Default to not trimming users on vacation.

chanserv.c (cmd_trim_users): New argument to indicate whether users on
    vacation should be included.
  (cmd_trim): Parse an extra option to control that argument.

chanserv.help: Document this change.
git-archimport-id: srvx@srvx.net--2005-srvx/srvx--devo--1.3--patch-35