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

src/backend/access/rmgrdesc/xactdesc.c

@@ -16,6 +16,7 @@
 
 #include "access/xact.h"
 #include "catalog/catalog.h"
+#include "common/relpath.h"
 #include "storage/sinval.h"
 #include "utils/timestamp.h"
 

src/backend/access/transam/xlogutils.c

@@ -20,6 +20,7 @@
 #include "access/xlog.h"
 #include "access/xlogutils.h"
 #include "catalog/catalog.h"
+#include "common/relpath.h"
 #include "storage/smgr.h"
 #include "utils/guc.h"
 #include "utils/hsearch.h"