BUG/MINOR: protocol: add missing support of dgram unix socket.

BUG/MINOR: protocol: add missing support of dgram unix socket.

The proto "uxdg" (UNIX DGRAM) was not declared, causing an error trying
to put a socket unix on "dgram-bind" into a log-forward section.

This patch introduces the missing "uxdg" protocol by adding proto_uxdg.c
which was fully created based on the code available for the other
protocols.

This patch should be backported to version 2.3 and above.

(cherry picked from commit 8af3bb0abf1b808fde2c944d768431377e6a1526)
Signed-off-by: Willy Tarreau <w@1wt.eu>