REORG: include: split common/htx.h into haproxy/htx{,-t}.h

Most of the file was a large set of HTX elements manipulation functions
and few types, so splitting them allowed to further reduce dependencies
and shrink the build time. Doing so revealed that a few files (h2.c,
mux_pt.c) needed haproxy/buf.h and were previously getting it through
htx.h. They were fixed.
diff --git a/include/proto/fcgi-app.h b/include/proto/fcgi-app.h
index aad0d61..424d018 100644
--- a/include/proto/fcgi-app.h
+++ b/include/proto/fcgi-app.h
@@ -22,7 +22,7 @@
 #ifndef _PROTO_HTTP_FCGI_H
 #define _PROTO_HTTP_FCGI_H
 
-#include <common/htx.h>
+#include <haproxy/htx.h>
 
 #include <types/fcgi-app.h>
 #include <types/proxy.h>