SPI function to investigate query semantics

Here's a patch containing the function SPI_iterate_query_roots(...). I'm
optimistic so it's complete with documentation :-)

I think that this function is needed so that PL/<lang> authors like
myself have a way to investigate the semantics of a prepared query. For
me this is essential since I want to prevent that savepoint related
statements are executed using normal SQL so that I can enforce the use
of the methods stipulated by the connection interface.

I forsee that this might be of interest for other PL/<lang> authors as
well. With this patch in place, it will be possible to do things like
this (returning false is rejecting in this case since false terminates
the iteration):

static bool rejectTransactionCommand(Query* query, void* clientData)
{
return !(query->commandType == CMD_UTILITY &&
IsA(query->utilityStmt, TransactionStmt));
}

and then use that like:

result = !SPI_iterate_query_roots(ePlan, rejectTransactionCommand,
NULL);

The patch has no side effects since it's a pure addon.

Kind regards,
Thomas Hallgren