REORG: split "protocols" files into protocol and listener

It was becoming confusing to have protocols and listeners in the same
files, split them.
diff --git a/src/backend.c b/src/backend.c
index c2ecda8..29d8145 100644
--- a/src/backend.c
+++ b/src/backend.c
@@ -38,7 +38,7 @@
 #include <proto/lb_fwlc.h>
 #include <proto/lb_fwrr.h>
 #include <proto/lb_map.h>
-#include <proto/protocols.h>
+#include <proto/protocol.h>
 #include <proto/proto_http.h>
 #include <proto/proto_tcp.h>
 #include <proto/queue.h>