[8.4] Updated WITH clause patch (non-recursive)

Attached is an updated version of Greg Stark's patch to add support for
the non-recursive variant of the SQL99 WITH clause[1]. I haven't looked
at the actual functionality of the patch yet (which is quite trivial) --
I just fixed up bitrot and the like. I also removed support for
RECURSIVE and the search/cycle clause, along with their associated
keywords -- the current patch doesn't approach anything close to adding
support for the non-recursive case, so it seems like a net loss to add
additional keywords for no gain in functionality.

Remaining work is to review the guts of the patch (which shouldn't take
long), and write documentation and regression tests. I'm personally
hoping to see this get into the tree fairly early in the 8.4 cycle,
pending discussion of course.

-Neil

[1] http://archives.postgresql.org/pgsql-patches/2007-03/msg00139.php
    http://archives.postgresql.org/pgsql-patches/2007-04/msg00055.php