Allow IMPORT FOREIGN SCHEMA within pl/pgsql.

Since IMPORT FOREIGN SCHEMA has an INTO clause, pl/pgsql needs to be
aware of that and avoid capturing the INTO as an INTO-variables clause.
This isn't hard, though it's annoying to have to make IMPORT a plpgsql
keyword just for this.  (Fortunately, we have the infrastructure now
to make it an unreserved keyword, so at least this shouldn't break any
existing pl/pgsql code.)

Per report from Merlin Moncure.  Back-patch to 9.5 where IMPORT FOREIGN
SCHEMA was introduced.

Report: <CAHyXU0wpHf2bbtKGL1gtUEFATCY86r=VKxfcACVcTMQ70mCyig@mail.gmail.com>