um: add a pseudo RTC

Add a pseudo RTC that simply is able to send an alarm signal
waking up the system at a given time in the future.

Since apparently timerfd_create() FDs don't support SIGIO, we
use the sigio-creating helper thread, which just learned to do
suspend/resume properly in the previous patch.

For time-travel mode, OTOH, just add an event at the specified
time in the future, and that's already sufficient to wake up
the system at that point in time since suspend will just be in
an "endless wait".

For s2idle support also call pm_system_wakeup().

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Richard Weinberger <richard@nod.at>