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/Makefile b/Makefile
index 53136be..681fe39 100644
--- a/Makefile
+++ b/Makefile
@@ -816,7 +816,7 @@
        src/buffer.o src/uri_auth.o src/protocol.o src/auth.o                  \
        src/ebsttree.o src/pipe.o src/hpack-enc.o src/fcgi.o                   \
        src/eb64tree.o src/dict.o src/shctx.o src/ebimtree.o                   \
-       src/eb32tree.o src/ebtree.o src/proto_udp.o                            \
+       src/eb32tree.o src/ebtree.o src/dgram.o                                \
        src/hpack-huff.o src/base64.o src/version.o
 
 ifneq ($(TRACE),)