Add --rsync-only option

Sometimes it's desirable to re-sync a "stale" data directory
on a standby, rather than start from scratch with pg_basebackup().

This re-adds the rsync code from the 2.x series, with some
modifications.

TODO: tablespace support.

dbutils.h

@@ -55,4 +55,7 @@ char       *get_repmgr_schema(void);
 char       *get_repmgr_schema_quoted(PGconn *conn);
 bool		create_replication_slot(PGconn *conn, char *slot_name);
 
+bool		start_backup(PGconn *conn, char *first_wal_segment);
+bool		stop_backup(PGconn *conn, char *last_wal_segment);
+
 #endif