Delete any replication slots copied by pg_rewind

If --force-rewind is used in conjunction with "repmgr node rejoin",
any replication slots present on the source node will be copied too;
it's essential to remove these to prevent stale slots being extant
when the node starts up.

We do this at file system level *before* the server starts to minimize
the risk of any problems.

Addresses GitHub #334

dirutil.c

@@ -311,6 +311,14 @@ create_pg_dir(char *path, bool force)
 	return true;
 }
 
+
+
+int
+rmdir_recursive(char *path)
+{
+	return nftw(path, unlink_dir_callback, 64, FTW_DEPTH | FTW_PHYS);
+}
+
 static int
 unlink_dir_callback(const char *fpath, const struct stat *sb, int typeflag, struct FTW *ftwbuf)
 {