Allow an optional alias for the target table to be specified for UPDATE

and DELETE. If specified, the alias must be used instead of the full
table name. Also, the alias currently cannot be used in the SET clause
of UPDATE.

Patch from Atsushi Ogawa, various editorialization by Neil Conway.
Along the way, make the rowtypes regression test pass if add_missing_from
is enabled, and add a new (skeletal) regression test for DELETE.