Fix for FETCH FIRST syntax problems

Per bug #15200, our support for sql2008 FETCH FIRST syntax is incomplete
to the extent that it should be regarded as a bug; the spec quite
clearly allows parameters/host variables in addition to constants, but
we don't.

Attached is a draft patch for fixing that, which additionally fixes the
ugly wart that OFFSET <x> ROW/ROWS and FETCH FIRST [<x>] ROW/ROWS ONLY
had very different productions for <x>; both now accept c_expr there.

Shift/reduce conflict is avoided by taking advantage of the fact that
ONLY is a fully reserved word.

I've checked that this change would not make it any harder to add
(post-2008 features) WITH TIES or PERCENT in the event that someone
wants to do that.

I think a case can be made that this should be backpatched; thoughts?

(While I can't find any visible change for existing working queries, one
change that does occur is that FETCH FIRST -1 ROWS ONLY now returns a
different error message; but that was already inconsistent with the
error from OFFSET -1 ROWS.)

-- 
Andrew (irc:RhodiumToad)