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/commands/tablespace.c

@@ -64,6 +64,7 @@
 #include "commands/comment.h"
 #include "commands/seclabel.h"
 #include "commands/tablespace.h"
+#include "common/relpath.h"
 #include "miscadmin.h"
 #include "postmaster/bgwriter.h"
 #include "storage/fd.h"