Don't allow write to /dev/rtc0, and remove sys_time.

Thanks, Christoph.

v2: drop sys_time, sys_module, mac_admin and mac_override in
all templates.
Reported-by: Christoph Mitasch <cmitasch@thomas-krenn.com>
Signed-off-by: Serge Hallyn <serge.hallyn@ubuntu.com>