Do some minor code refactoring in preparation for changing the APIs of

find_inheritance_children() and find_all_inheritors().  I got annoyed that
these are buried inside the planner but mostly used elsewhere.  So, create
a new file catalog/pg_inherits.c and put them there, along with a couple
of other functions that search pg_inherits.

The code that modifies pg_inherits is (still) in tablecmds.c --- it's
kind of entangled with unrelated code that modifies pg_depend and other
stuff, so pulling it out seemed like a bigger change than I wanted to make
right now.  But this file provides a natural home for it if anyone ever
gets around to that.

This commit just moves code around; it doesn't change anything, except
I succumbed to the temptation to make a couple of trivial optimizations
in typeInheritsFrom().

src/backend/commands/lockcmds.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/commands/lockcmds.c,v 1.21 2009/01/12 08:54:26 petere Exp $
+ *	  $PostgreSQL: pgsql/src/backend/commands/lockcmds.c,v 1.22 2009/05/12 00:56:05 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -16,9 +16,9 @@
 
 #include "access/heapam.h"
 #include "catalog/namespace.h"
+#include "catalog/pg_inherits.h"
 #include "commands/lockcmds.h"
 #include "miscadmin.h"
-#include "optimizer/prep.h"
 #include "parser/parse_clause.h"
 #include "utils/acl.h"
 #include "utils/lsyscache.h"