[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.
diff --git a/doc/configuration.txt b/doc/configuration.txt
index c9ba70d..e26fcb6 100644
--- a/doc/configuration.txt
+++ b/doc/configuration.txt
@@ -84,10 +84,20 @@
 log <address> <facility> [max level]
   Adds a global syslog server. Up to two global servers can be defined. They
   will receive logs for startups and exits, as well as all logs from proxies
-  configured with "log global". <address> is an IPv4 address optionally
-  followed by a colon and an UDP port. If no port is specified, 514 is used
-  by default (the standard syslog port). <facility> must be one of the 24
-  standard syslog facilities :
+  configured with "log global".
+
+  <address> can be one of:
+
+        - An IPv4 address optionally followed by a colon and an UDP port. If
+          no port is specified, 514 is used by default (the standard syslog
+          port).
+
+        - A filesystem path to a UNIX domain socket, keeping in mind
+          considerations for chroot (be sure the path is accessible inside
+          the chroot) and uid/gid (be sure the path is appropriately
+          writeable).
+
+  <facility> must be one of the 24 standard syslog facilities :
 
           kern   user   mail   daemon auth   syslog lpr    news
           uucp   cron   auth2  ftp    ntp    audit  alert  cron2