pl/pgsql: END verbosity

In PL/PgSQL, "END LOOP" is used to terminate loop blocks, and "END IF"
is used to terminate IF blocks. This is needlessly verbose: we could
simply accept "END" in both cases without syntactic ambiguity. I'd like
to make this change, so that END can be used to terminate any kind of
block. There's no need to remove support for the present syntax, of
course, so there's no backward compatibility concern. Oracle's PL/SQL
does require "END IF" and "END LOOP", but folks interested in maximum
compatibility can always use those forms if they like.

Any objections?

-Neil