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/checks.c b/src/checks.c
index 2b7aa8c..4a1b87f 100644
--- a/src/checks.c
+++ b/src/checks.c
@@ -59,7 +59,7 @@
 #include <proto/queue.h>
 #include <proto/port_range.h>
 #include <proto/proto_tcp.h>
-#include <proto/protocol.h>
+#include <haproxy/protocol.h>
 #include <proto/proxy.h>
 #include <proto/server.h>
 #include <proto/signal.h>