REORG: include: move protocol.h to haproxy/protocol{,-t}.h

The protocol.h files are pretty low in the dependency and (sadly) used
by some files from common/. Almost nothing was changed except lifting a
few comments.
diff --git a/src/protocol.c b/src/protocol.c
index 4909ee1..defb197 100644
--- a/src/protocol.c
+++ b/src/protocol.c
@@ -18,7 +18,7 @@
 #include <haproxy/list.h>
 #include <common/standard.h>
 
-#include <proto/protocol.h>
+#include <haproxy/protocol.h>
 
 /* List head of all registered protocols */
 static struct list protocols = LIST_HEAD_INIT(protocols);