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.
diff --git a/Makefile b/Makefile
index 0867047..ab3ad48 100644
--- a/Makefile
+++ b/Makefile
@@ -878,7 +878,7 @@
         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/fix.o src/mqtt.o src/dns.o    \
-        src/server_state.o
+        src/server_state.o src/proto_uxdg.o
 
 ifneq ($(TRACE),)
 OBJS += src/calltrace.o