doc: Mention REPACK in MAINTAIN privilege descriptions

doc: Mention REPACK in MAINTAIN privilege descriptions

REPACK requires the MAINTAIN privilege, but it was omitted from the
lists of commands covered by that privilege in ddl.sgml and the
description of the predefined pg_maintain role in user-manag.sgml.
This was an oversight in commit ac58465e061, which introduced
REPACK.

Add REPACK to both documentation lists, and update the corresponding
comment in aclchk.c.

Author: Shinya Kato <shinya11.kato@gmail.com>
Reviewed-by: Ewan Young <kdbase.hack@gmail.com>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Discussion: https://postgr.es/m/CAOzEurRJOVokiB2J8nrF569nX-ZMb0oRSB0C=yZQ17mZxd4_BQ@mail.gmail.com
Backpatch-through: 19