transparent proxy: use IP_BINDANY where available

[dlundquist]

This should add transparent proxy support for FreeBSD. I've tested this builds on FreeBSD 12.0, but I'm not sure if there are similar firewall considerations as there in on Linux:

sniproxy/tests/transparent_proxy_test

Lines 101 to 157 in 87461aa

	# Setup a test network using network namespaces with the ns-test-proxy
	# namespace configured like a normal transparent proxy box.
	#
	# +-----------------------------------------------------------+
	# | Host (default) Network Namespace |
	# | |
	# | Route 192.0.2.4/30 via 192.0.2.1 dev veth-srv-proxy |
	# | |
	# | TestHTTPD process o veth-srv-proxy |
	# | | 192.0.2.2/30 |
	# | +-----------------------------------|---------------+ |
	# | | SNIproxy Network Namespace | | |
	# | | ns-test-proxy o veth-proxy-srv| |
	# | | 192.0.2.1/30 | |
	# | | IP Forwarding enabled | |
	# | | IPTables and ip rules configured (see below) | |
	# | | | |
	# | | o veth-proxy-clt| |
	# | | sniproxy process | 192.0.2.5/30 | |
	# | | | | |
	# | +-----------------------------------|---------------+ |
	# | | |
	# | +-----------------------------------|---------------+ |
	# | | Client Network Namespace | veth-clt-proxy| |
	# | | ns-test-clt o 192.0.2.6/30 | |
	# | | | |
	# | | Route default via 192.0.2.5 dev veth-clt-proxy | |
	# | | | |
	# | | curl process | |
	# | | | |
	# | +---------------------------------------------------+ |
	# | |
	# +-----------------------------------------------------------+
	#
	# IPTables NAT rules:
	# Chain POSTROUTING (policy ACCEPT)
	# target prot opt source destination
	# MASQUERADE all -- 0.0.0.0/0 0.0.0.0/0
	#
	# IPTables Mangle rules:
	# Chain PREROUTING (policy ACCEPT)
	# target prot opt source destination
	# DIVERT tcp -- 0.0.0.0/0 0.0.0.0/0 socket
	#
	# Chain DIVERT (1 references)
	# target prot opt source destination
	# MARK all -- 0.0.0.0/0 0.0.0.0/0 MARK set 0x1
	# ACCEPT all -- 0.0.0.0/0 0.0.0.0/0
	#
	# IP rules:
	# 0: from all lookup local
	# 32765: from all fwmark 0x1 lookup 100
	# 32766: from all lookup main
	# 32767: from all lookup default
	#
	# IP route table 100:
	# local default dev lo scope host

. If someone wants to make transparent_proxy_test work on FreeBSD too that would be great.

See issue #315.