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>
diff --git a/Makefile b/Makefile
index 9496a6f..e13f497 100644
--- a/Makefile
+++ b/Makefile
@@ -857,7 +857,7 @@
         src/ebimtree.o src/uri_auth.o src/freq_ctr.o src/ebsttree.o            \
         src/ebistree.o src/auth.o src/wdt.o src/http_acl.o                     \
         src/hpack-enc.o src/hpack-huff.o src/ebtree.o src/base64.o             \
-        src/hash.o src/dgram.o src/version.o
+        src/hash.o src/dgram.o src/version.o src/proto_uxdg.o
 
 ifneq ($(TRACE),)
 OBJS += src/calltrace.o