psql: Fix tab completion for VACUUM option values.

psql: Fix tab completion for VACUUM option values.

Commit 8a3e4011 introduced tab completion for the ONLY option of
VACUUM and ANALYZE, along with some code simplification using
MatchAnyN. However, it caused a regression in tab completion for
VACUUM option values. For example, neither ON nor OFF was suggested
after "VACUUM (VERBOSE". In addition, the ONLY keyword was not
suggested immediately after a completed option list.

Backpatch to v18.

Author: Yugo Nagata <nagata@sraoss.co.jp>
Discussion: https://postgr.es/m/20251223021509.19bba68ecbbc70c9f983c2b4@sraoss.co.jp
Backpatch-through: 18