MEDIUM: protocol: add minimalist UDP protocol client

Basic introduction of a UDP layer in HAProxy. It can be used as a
client only and manages UDP exchanges with servers.

It can't be used to load-balance UDP protocols, but only used by
internal features such as DNS resolution.
diff --git a/Makefile b/Makefile
index 10de1de..2059da9 100644
--- a/Makefile
+++ b/Makefile
@@ -731,7 +731,7 @@
        src/lb_chash.o src/lb_fwlc.o src/lb_fwrr.o src/lb_map.o src/lb_fas.o \
        src/stream_interface.o src/dumpstats.o src/proto_tcp.o src/applet.o \
        src/session.o src/stream.o src/hdr_idx.o src/ev_select.o src/signal.o \
-       src/acl.o src/sample.o src/memory.o src/freq_ctr.o src/auth.o \
+       src/acl.o src/sample.o src/memory.o src/freq_ctr.o src/auth.o src/proto_udp.o \
        src/compression.o src/payload.o src/hash.o src/pattern.o src/map.o \
        src/namespace.o src/mailers.o