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"

src/backend/storage/buffer/localbuf.c

@@ -16,6 +16,7 @@
 #include "postgres.h"
 
 #include "catalog/catalog.h"
+#include "common/relpath.h"
 #include "executor/instrument.h"
 #include "storage/buf_internals.h"
 #include "storage/bufmgr.h"

src/backend/storage/file/fd.c

@@ -71,6 +71,7 @@
 #include "access/xact.h"
 #include "catalog/catalog.h"
 #include "catalog/pg_tablespace.h"
+#include "common/relpath.h"
 #include "pgstat.h"
 #include "storage/fd.h"
 #include "storage/ipc.h"

src/backend/storage/file/reinit.c

@@ -17,6 +17,7 @@
 #include <unistd.h>
 
 #include "catalog/catalog.h"
+#include "common/relpath.h"
 #include "storage/copydir.h"
 #include "storage/fd.h"
 #include "storage/reinit.h"

src/backend/storage/smgr/md.c

@@ -21,6 +21,7 @@
 #include "miscadmin.h"
 #include "access/xlog.h"
 #include "catalog/catalog.h"
+#include "common/relpath.h"
 #include "portability/instr_time.h"
 #include "postmaster/bgwriter.h"
 #include "storage/fd.h"