Fix tab completion for ALTER ROLE|USER ... RESET

Fix tab completion for ALTER ROLE|USER ... RESET

Commit c407d5426b87 added tab completion for ALTER ROLE|USER ... RESET,
with the intent to offer only the variables actually set on the role.
But as soon as the user started typing something, it would start to
offer all possible matching variables.

Fix this the same way ALTER DATABASE ... RESET does it, i.e. by
properly considering the prefix.

A second issue causing similar symptoms (offering variables that are not
actually set for a role) was caused by a match to another pattern. The
ALTER DATABASE ... RESET was already excluded, so do the same thing for
ROLE/USER.

Report and fix by Dagfinn Ilmari Mannsåker. Backpatch to 18, same as
c407d5426b87.

Author: Dagfinn Ilmari Mannsåker <ilmari@ilmari.org>
Reviewed-by: jian he <jian.universality@gmail.com>
Discussion: https://postgr.es/m/87qzyghw2x.fsf%40wibble.ilmari.org
Discussion: https://postgr.es/m/87tt4lumqz.fsf%40wibble.ilmari.org
Backpatch-through: 18