Various small revisions to TCPEndPoint

[kpschoedel]

Problem

Working toward #7715 Virtualize System and Inet interfaces

Change overview

Miscellaneous revisions with no change to functionality.

• Add TCPTest class for test access to internal TCPEndPoint state.
• Make State private.
• Favor ReturnErrorOnFailure() etc.
• Factor GetSocketInfo() out of Get(Local|Peer)Info()

Testing

CI; no changes to functionality intended.

[woody-apple]

@saurabhst @msandstedt @mspang @andy31415 @Damian-Nordic ?

[github-actions]

PR #10864: Size comparison from e282eec to 516d176

35 builds (for efr32, esp32, k32w, linux, mbed, nrfconnect, p6, qpg, telink)

platform	target	config	section	e282eec	516d176	change	% change
efr32	lighting-app	BRD4161A	.bss	118036	118036	0	0.0
			.data	1800	1800	0	0.0
			.text	784296	784344	48	0.0
	lock-app	BRD4161A	.bss	115884	115884	0	0.0
			.data	1760	1760	0	0.0
			.text	763440	763488	48	0.0
	window-app	BRD4161A	.bss	116204	116204	0	0.0
			.data	1764	1764	0	0.0
			.text	764356	764404	48	0.0
	lighting-app	BRD4161A+rpc	.bss	131364	131364	0	0.0
			.data	1852	1852	0	0.0
			.text	764012	764060	48	0.0
esp32	all-clusters-app	c3devkit	.dram0.bss	60296	60296	0	0.0
			.dram0.data	16440	16440	0	0.0
			.flash.rodata	199912	199912	0	0.0
			.flash.text	874326	874326	0	0.0
			.iram0.text	57564	57564	0	0.0
		m5stack	.dram0.bss	62808	62808	0	0.0
			.dram0.data	32084	32084	0	0.0
			.flash.rodata	208648	208648	0	0.0
			.flash.text	905335	905335	0	0.0
			.iram0.text	125115	125115	0	0.0
k32w	lock-app	k32w061+debug	.bss	69068	69068	0	0.0
			.data	1864	1864	0	0.0
			.text	516024	516024	0	0.0
	shell	k32w061+debug	.bss	55080	55080	0	0.0
			.data	672	672	0	0.0
			.text	359480	359480	0	0.0
	lighting-app	k32w061+se05x+release	.bss	78592	78592	0	0.0
			.data	1900	1900	0	0.0
			.text	614788	614788	0	0.0
linux	all-clusters-app	debug	.bss	52208	52208	0	0.0
			.data	978	978	0	0.0
			.data.rel.ro	60384	60384	0	0.0
			.dynamic	592	592	0	0.0
			.got	4088	4088	0	0.0
			.init	27	27	0	0.0
			.init_array	512	512	0	0.0
			.rodata	136437	136437	0	0.0
			.text	1352610	1352626	16	0.0
	chip-tool	debug	.bss	17712	17712	0	0.0
			.data	1584	1584	0	0.0
			.data.rel.ro	93584	93584	0	0.0
			.dynamic	592	592	0	0.0
			.got	4368	4368	0	0.0
			.init	27	27	0	0.0
			.init_array	416	416	0	0.0
			.rodata	194148	194148	0	0.0
			.text	3506501	3506533	32	0.0
	ota-provider-app	debug	.bss	37472	37472	0	0.0
			.data	752	752	0	0.0
			.data.rel.ro	24488	24488	0	0.0
			.dynamic	592	592	0	0.0
			.got	4016	4016	0	0.0
			.init	27	27	0	0.0
			.init_array	440	440	0	0.0
			.rodata	110312	110312	0	0.0
			.text	1022578	1022594	16	0.0
	ota-requestor-app	debug	.bss	205728	205728	0	0.0
			.data	752	752	0	0.0
			.data.rel.ro	25800	25800	0	0.0
			.dynamic	592	592	0	0.0
			.got	4144	4144	0	0.0
			.init	27	27	0	0.0
			.init_array	512	512	0	0.0
			.rodata	128392	128392	0	0.0
			.text	1141106	1141122	16	0.0
	shell	debug	.bss	16136	16136	0	0.0
			.data	242	242	0	0.0
			.data.rel.ro	36496	36496	0	0.0
			.dynamic	592	592	0	0.0
			.got	3528	3528	0	0.0
			.init	27	27	0	0.0
			.init_array	336	336	0	0.0
			.rodata	76463	76463	0	0.0
			.text	598242	598098	-144	-0.0
	tv-app	debug	.bss	216592	216592	0	0.0
			.data	2032	2032	0	0.0
			.data.rel.ro	57408	57408	0	0.0
			.dynamic	592	592	0	0.0
			.got	4408	4408	0	0.0
			.init	27	27	0	0.0
			.init_array	608	608	0	0.0
			.rodata	152936	152936	0	0.0
			.text	1452994	1453010	16	0.0
	bridge-app	debug+rpc	.bss	52912	52912	0	0.0
			.data	976	976	0	0.0
			.data.rel.ro	27112	27112	0	0.0
			.dynamic	592	592	0	0.0
			.got	3952	3952	0	0.0
			.init	27	27	0	0.0
			.init_array	400	400	0	0.0
			.rodata	110748	110748	0	0.0
			.text	1063461	1063493	32	0.0
	lighting-app	debug+rpc	.bss	42232	42232	0	0.0
			.data	1106	1106	0	0.0
			.data.rel.ro	53808	53808	0	0.0
			.dynamic	608	608	0	0.0
			.got	4112	4112	0	0.0
			.init	27	27	0	0.0
			.init_array	528	528	0	0.0
			.rodata	127921	127921	0	0.0
			.text	1261410	1261442	32	0.0
mbed	lighting-app	CY8CPROTO_062_4343W+release	.bss	172100	172100	0	0.0
			.data	5464	5464	0	0.0
			.heap	858880	858880	0	0.0
			.text	1220088	1220088	0	0.0
	lock-app	CY8CPROTO_062_4343W+release	.bss	171020	171020	0	0.0
			.data	5432	5432	0	0.0
			.heap	859992	859992	0	0.0
			.text	1197992	1197992	0	0.0
	pigweed-app	CY8CPROTO_062_4343W+release	.bss	11760	11760	0	0.0
			.data	4360	4360	0	0.0
			.heap	1020328	1020328	0	0.0
			.text	103064	103064	0	0.0
nrfconnect	lighting-app	nrf52840dk_nrf52840	bss	113180	113180	0	0.0
			rodata	98080	98080	0	0.0
			text	577104	577104	0	0.0
	lock-app	nrf52840dk_nrf52840	bss	112244	112244	0	0.0
			rodata	94416	94416	0	0.0
			text	558592	558592	0	0.0
	pigweed-app	nrf52840dk_nrf52840	bss	51824	51824	0	0.0
			rodata	45776	45776	0	0.0
			text	339456	339456	0	0.0
	pump-app	nrf52840dk_nrf52840	bss	112316	112316	0	0.0
			rodata	95396	95396	0	0.0
			text	561744	561744	0	0.0
	pump-controller-app	nrf52840dk_nrf52840	bss	112252	112252	0	0.0
			rodata	94476	94476	0	0.0
			text	558384	558384	0	0.0
	shell	nrf52840dk_nrf52840	bss	109072	109072	0	0.0
			rodata	72524	72524	0	0.0
			text	520208	520208	0	0.0
	lighting-app	nrf52840dk_nrf52840+rpc	bss	109420	109420	0	0.0
			rodata	88856	88856	0	0.0
			text	550296	550296	0	0.0
		nrf5340dk_nrf5340_cpuapp	bss	114552	114552	0	0.0
			rodata	93320	93320	0	0.0
			text	506572	506572	0	0.0
	lock-app	nrf5340dk_nrf5340_cpuapp	bss	113620	113620	0	0.0
			rodata	89676	89676	0	0.0
			text	488056	488056	0	0.0
	shell	nrf5340dk_nrf5340_cpuapp	bss	110056	110056	0	0.0
			rodata	67168	67168	0	0.0
			text	440816	440816	0	0.0
p6	lock-app	default	.bss	68216	68216	0	0.0
			.data	2416	2416	0	0.0
			.heap	962712	962712	0	0.0
			.text	1126688	1126736	48	0.0
qpg	lighting-app	qpg6100+debug	.bss	53464	53464	0	0.0
			.data	1004	1004	0	0.0
			.text	486036	486036	0	0.0
	lock-app	qpg6100+debug	.bss	52408	52408	0	0.0
			.data	960	960	0	0.0
			.text	462120	462120	0	0.0
	persistent-storage-app	qpg6100+debug	.bss	27752	27752	0	0.0
			.data	372	372	0	0.0
			.text	149896	149896	0	0.0
telink	lighting-app	tlsr9518adk80d	bss	71000	71000	0	0.0
			noinit	33216	33216	0	0.0
			text	457612	457612	0	0.0