Improve psql's tab completion for ALTER EXTENSION foo UPDATE ...

Offer a list of available versions for that extension.  Formerly, since
there was no special support for this, it triggered off the UPDATE
keyword and offered a list of table names --- not too helpful.

Jeff Janes, reviewed by Gerdan Santos

Patch: <CAMkU=1z0gxEOLg2BWa69P4X4Ot8xBxipGUiGkXe_tC+raj79-Q@mail.gmail.com>