dbus: create dbus:22 system account.
avahi: create avahi:23 system account.
base-files: remove avahi/dbus/systemd-journal from /etc/{passwd,group}.
xbps-triggers: system-accounts: accept acct:uid.
hooks/xbps-metadata-scripts: add support for system_accounts=foo:uid.
xbps-triggers: system-accounts: simplify creation.