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/storage/buffer/bufmgr.c

@@ -34,6 +34,7 @@
 #include <unistd.h>
 
 #include "catalog/catalog.h"
+#include "common/relpath.h"
 #include "executor/instrument.h"
 #include "miscadmin.h"
 #include "pg_trace.h"