[MEDIUM] add support for logging via a UNIX socket

The code in haproxy-1.3.13.1 only supports syslogging to an internet
address. The attached patch:

 - Adds support for syslogging to a UNIX domain socket (e.g., /dev/log).
   If the address field begins with '/' (absolute file path), then
   AF_UNIX is used to construct the socket. Otherwise, AF_INET is used.

 - Achieves clean single-source build on both Mac OS X and Linux
   (sockaddr_in.sin_len and sockaddr_un.sun_len field aren't always present).

For handling sendto() failures in send_log(), it appears that the existing
code is fine (no need to close/recreate socket) for both UDP and UNIX-domain
syslog server. So I left things alone (did not close/recreate socket).
Closing/recreating socket after each failure would also work, but would lead
to increased amount of unnecessary socket creation/destruction if syslog is
temporarily unavailable for some reason (especially for verbose loggers).

Please consider this patch for inclusion into the upstream haproxy codebase.
8 files changed