patch to replace VARATT_SIZEP macro

Here's a patch to remove the VARATT_SIZEP macro replacing it with
SET_VARLENA_LEN. It doesn't change the actual format of the size header so
it's basically a noop in terms of functionality.

I did add an AssertMacro to check the length is reasonable, but that's the
only functional difference.

If anybody has any major patches getting near to being committed in the next
few days then it probably makes sense to hold this until they hit CVS. I would
be happy to update it when that happens.

However if that's not the case it makes sense to commit this asap as it will
bitrot rapidly. Everyone should be sure to update their cvs tree before
starting any new work after this gets committed.