Move relpath() to libpgcommon

This enables non-backend code, such as pg_xlogdump, to use it easily.
The previous location, in src/backend/catalog/catalog.c, made that
essentially impossible because that file depends on many backend-only
facilities; so this needs to live separately.

src/backend/access/rmgrdesc/smgrdesc.c

@@ -16,6 +16,7 @@
 
 #include "catalog/catalog.h"
 #include "catalog/storage_xlog.h"
+#include "common/relpath.h"
 
 
 void