Check data directory not used by an active instance before cloning

This involves adding some infrastructure to parse pg_control in a
reasonably version-independent fashion. This will probably be useful
in other contexts where we need to verify the status of a
data directory where the instance isn't running.

controldata.h

@@ -0,0 +1,27 @@
+/*
+ * controldata.h
+ * Copyright (c) 2ndQuadrant, 2010-2017
+ *
+ * Portions Copyright (c) 1996-2016, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ */
+
+#ifndef _CONTROLDATA_H_
+#define _CONTROLDATA_H_
+
+#include "postgres_fe.h"
+#include "catalog/pg_control.h"
+
+typedef struct
+{
+	bool control_file_processed;
+	ControlFileData *control_file;
+} ControlFileInfo;
+
+extern DBState
+get_db_state(char *data_directory);
+
+extern const char *
+describe_db_state(DBState state);
+
+#endif /* _CONTROLDATA_H_ */