Add postgres_fdw contrib module.

There's still a lot of room for improvement, but it basically works,
and we need this to be present before we can do anything much with the
writable-foreign-tables patch.  So let's commit it and get on with testing.

Shigeru Hanada, reviewed by KaiGai Kohei and Tom Lane

contrib/postgres_fdw/postgres_fdw.h

@@ -0,0 +1,52 @@
+/*-------------------------------------------------------------------------
+ *
+ * postgres_fdw.h
+ *		  Foreign-data wrapper for remote PostgreSQL servers
+ *
+ * Portions Copyright (c) 2012-2013, PostgreSQL Global Development Group
+ *
+ * IDENTIFICATION
+ *		  contrib/postgres_fdw/postgres_fdw.h
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef POSTGRES_FDW_H
+#define POSTGRES_FDW_H
+
+#include "foreign/foreign.h"
+#include "lib/stringinfo.h"
+#include "nodes/relation.h"
+#include "utils/rel.h"
+
+#include "libpq-fe.h"
+
+/* in connection.c */
+extern PGconn *GetConnection(ForeignServer *server, UserMapping *user);
+extern void ReleaseConnection(PGconn *conn);
+extern unsigned int GetCursorNumber(PGconn *conn);
+extern void pgfdw_report_error(int elevel, PGresult *res, bool clear,
+				   const char *sql);
+
+/* in option.c */
+extern int ExtractConnectionOptions(List *defelems,
+						 const char **keywords,
+						 const char **values);
+
+/* in deparse.c */
+extern void classifyConditions(PlannerInfo *root,
+				   RelOptInfo *baserel,
+				   List **remote_conds,
+				   List **param_conds,
+				   List **local_conds,
+				   List **param_numbers);
+extern void deparseSimpleSql(StringInfo buf,
+				 PlannerInfo *root,
+				 RelOptInfo *baserel,
+				 List *local_conds);
+extern void appendWhereClause(StringInfo buf,
+				  bool has_where,
+				  List *exprs,
+				  PlannerInfo *root);
+extern void deparseAnalyzeSql(StringInfo buf, Relation rel);
+
+#endif   /* POSTGRES_FDW_H */