Constant text strings actually take up SRAM space on the ESP8266
because the .RODATA segment must be copied to RAM at startup since
FLASH isn't byte-accessible.

Move the constant format strings in a printf completely into FLASH
and add a wrapper to copy it into a local stack-allocated space
when needed, freeing up about 3100 bytes of RAM for use.  This doesn't
make FLASH usage any higher, either, since those strings were already
being stored there (but never used after the power-on startup code).

Minor edits required in some of the output/debug/tracing functions,
but no logic changed.

ssl_ext has a hostname which is strdup()'d (i.e. allocated) but wasn't
freed in the ssl_ext_free() routine.  This lead to the leaking of that
strdup()'d pointer on every SSL close/reopen cycle.

Patch simply checks if there's a string and frees it in the ssl_ext_free
routine.  Also adds a sanity check in set_ext_hostname() to not try and
free() the old string if there is not one present.