Additional "standby clone" code

We'll break up the unwieldy "do_standby_clone()" function into discrete
unit for easier maintenance.

dirutil.h

@@ -0,0 +1,19 @@
+/*
+ * dirutil.h
+ * Copyright (c) 2ndQuadrant, 2010-2017
+ *
+ */
+
+#ifndef _DIRUTIL_H_
+#define _DIRUTIL_H_
+
+extern int		mkdir_p(char *path, mode_t omode);
+extern bool		set_dir_permissions(char *path);
+
+extern int		check_dir(char *path);
+extern bool		create_dir(char *path);
+extern bool		is_pg_dir(char *path);
+extern bool		create_pg_dir(char *path, bool force);
+extern bool		create_witness_pg_dir(char *path, bool force);
+
+#endif