Allow external recovery_config_directory

If required, recovery.conf can now be located outside of the data directory.
Server needs read/write permissions on this directory.

src/backend/utils/init/globals.c

@@ -46,6 +46,7 @@ int			MyPMChildSlot;
  * explicitly.
  */
 char	   *DataDir = NULL;
+char	   *RecoveryConfDir = NULL;
 
 char		OutputFileName[MAXPGPATH];	/* debugging output file */