sys-apps/smartmontools: fix update warning

sys-apps/smartmontools: fix update warning

- update warning about updating the drivedb, so that it notices the
    useflag
- fix description so that it's short enough (adapted from the
    smartmontools website) -- repoman
- quote some variables -- shellcheck
- update the copy of the warning

Fixes: https://bugs.gentoo.org/show_bug.cgi?id=587372
(cherry picked from commit 890093680d63a1b50d02e6408b55d4d748fcdbdb)
Signed-off-by: Robin H. Johnson <robbat2@gentoo.org>
Fixes: https://github.com/gentoo/gentoo/pull/3114