I copied over the rmtree function (and other functions needed by this one)

from the postgresql code so we use that instead of issuing system calls
with rm -rf ....

I also eliminated the rm -rf for pg_xlog.

Will later do the same with the other system call to remove files
in pg_replslot/

dirmod.h

@@ -0,0 +1,23 @@
+/*
+ * dirmod.h
+ * Copyright (c) 2ndQuadrant, 2010-2016
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ *
+ */
+
+#ifndef _DIRMOD_H_
+#define _DIRMOD_H_
+
+#endif