Harmonize password reuse in vacuumdb, clusterdb, and reindexdb.

Harmonize password reuse in vacuumdb, clusterdb, and reindexdb.

Commits 83dec5a712 and ff402ae11b taught vacuumdb to reuse
passwords instead of prompting repeatedly.  However, the docs still
warn about repeated prompts, and this improvement was not applied
to clusterdb and reindexdb.  This commit allows clusterdb and
reindexdb to reuse passwords just like vacuumdb does, and it
expunges the aforementioned warnings from the docs.

Reviewed-by: Gurjeet Singh, Zhang Mingli
Discussion: https://postgr.es/m/20230628045741.GA1813397%40nathanxps13