gai_strerror.[c3]: re-enable EAI_ADDRFAMILY, EAI_NODATA

gai_strerror.c still has messages for EAI_ADDRFAMILY and EAI_NODATA,
but not the man page. Re-add to the man page, and update comments
in the source. Document the errors that are not in RFC 3493 or
POSIX.

Reviewed in https://reviews.freebsd.org/D37139 with related changes.

Reviewed by: bz, pauamma
MFC after: 1 month