Writeable CTE patch

Hi,

Attached is the latest version of this patch.

I altered rewriting a bit (I've brought the problems with the previous 
approach up a couple of times before) and this version should have the 
expected output in all situations.  This patch doesn't allow you to use 
INSERT/UPDATE/DELETE as the top level statement, but you can get around 
that by putting the desired top-level statement in a new CTE.

Since the last patch I also moved ExecOpenIndices to nodeModifyTable.c 
because the top-level executor doesn't know which result relations are 
opened for which operations.

One thing which has bothered me a while is that there is no clear option 
for commandType when you have a multiple types of statements in a single 
Query.  In some places it'd help to know that there are multiple 
different statements.  This is now achieved by having hasWritableCtes 
variable in PlannedStmt, but that doesn't help in places where you don't 
have access to (or there isn't yet one) PlannedStmt, which has lead me 
to think that we could have a CMD_MULTI or a similar value to mark these 
Queries.  I haven't taken the time to look at this in detail, but it's 
something to think about.


Regards,
Marko Tiikkaja