Do honest transformation and preprocessing of LIMIT/OFFSET clauses,

Do honest transformation and preprocessing of LIMIT/OFFSET clauses,
instead of the former kluge whereby gram.y emitted already-transformed
expressions.  This is needed so that Params appearing in these clauses
actually work correctly.  I suppose some might claim that the side effect
of 'SELECT ... LIMIT 2+2' working is a new feature, but I say this is
a bug fix.