socks: fix build on FreeBSD

The fix should be sane on all UNIX-like systems, so there's
no ZMQ_HAVE_FREEBSD involved.  It's likely that other BSDs
stumble across this problem too.
Merge pull request #1113 from fichtner/socks_include

socks: fix build on FreeBSD
Add missing socks files
Merge pull request #1114 from PalmStoneGames/master

Add missing socks files