Add a SHOW command to the replication command language.

This is useful infrastructure for an upcoming proposed patch to
allow the WAL segment size to be changed at initdb time; tools like
pg_basebackup need the ability to interrogate the server setting.
But it also doesn't seem like a bad thing to have independently of
that; it may find other uses in the future.

Robert Haas and Beena Emerson.  (The original patch here was by
Beena, but I rewrote it to such a degree that most of the code
being committed here is mine.)

Discussion: http://postgr.es/m/CA+TgmobNo4qz06wHEmy9DszAre3dYx-WNhHSCbU9SAwf+9Ft6g@mail.gmail.com

src/include/access/tupdesc.h

@@ -119,6 +119,13 @@ extern void TupleDescInitEntry(TupleDesc desc,
 				   int32 typmod,
 				   int attdim);
 
+extern void TupleDescInitBuiltinEntry(TupleDesc desc,
+						  AttrNumber attributeNumber,
+						  const char *attributeName,
+						  Oid oidtypeid,
+						  int32 typmod,
+						  int attdim);
+
 extern void TupleDescInitEntryCollation(TupleDesc desc,
 							AttrNumber attributeNumber,
 							Oid collationid);