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

The type is the only element needed by applet.h and hlua.h, while hlua.c
needs the various functions. XREF_BUSY was placed into the types as well
since it's better to have the special values there.
diff --git a/include/types/applet.h b/include/types/applet.h
index 7b21f9a..d6b34a3 100644
--- a/include/types/applet.h
+++ b/include/types/applet.h
@@ -26,11 +26,11 @@
 #include <haproxy/buf.h>
 #include <haproxy/dynbuf-t.h>
 #include <haproxy/freq_ctr-t.h>
+#include <haproxy/xref-t.h>
 #include <types/hlua.h>
 #include <types/obj_type.h>
 #include <types/proxy.h>
 #include <types/stream.h>
-#include <common/xref.h>
 
 struct appctx;