rc.subr: add 'settime' to svcj options

<svc>_svcj_options="settime" enables the jail allow.settime privilege,
which allows to set and slew the system clock. this allows NTP daemons
to run in a service jail.

Reviewed by: jamie, kevans, des, Jails, manpages
Approved by: kevans (mentor)
Differential Revision: https://reviews.freebsd.org/D49845