Create a validator for plpgsql, so that some minimal syntax checking

Create a validator for plpgsql, so that some minimal syntax checking
is done at creation time for plpgsql functions.  Improve createlang and
droplang to support adding/dropping validators for PLs.  Initial steps
towards producing a syntax error position from plpgsql syntax errors
(this part is a work in progress, and will change depending on outcome
of current discussions).