REORG: dgram: rename proto_udp to dgram

The set of files proto_udp.{c,h} were misleadingly named, as they do not
provide anything related to the UDP protocol but to datagram handling
instead, since currently all UDP processing is hard-coded where it's used
(dns, logs). They are to UDP what connection.{c,h} are to proto_tcp. This
was causing confusion about how to insert UDP socket management code,
so let's rename them right now to dgram.{c,h} which more accurately
matches what's inside since every function and type is already prefixed
with "dgram_".
diff --git a/src/tools.c b/src/tools.c
index 0b7e77e..35936b5 100644
--- a/src/tools.c
+++ b/src/tools.c
@@ -41,12 +41,12 @@
 
 #include <haproxy/api.h>
 #include <haproxy/chunk.h>
+#include <haproxy/dgram.h>
 #include <haproxy/dns.h>
 #include <haproxy/global.h>
 #include <haproxy/hlua.h>
 #include <haproxy/listener.h>
 #include <haproxy/namespace.h>
-#include <haproxy/proto_udp.h>
 #include <haproxy/ssl_sock.h>
 #include <haproxy/stream_interface.h>
 #include <haproxy/task.h>