Add a 'power' unit

This unit responds to logind's attempts to poweroff, reboot, hibernate
and suspend.

Suspend and hibernate are implemented via pm-utils if it is installed or
via the kernel directly otherwise.

Take care to note when a reboot or shutdown is requested and ignore
suspend/hibernate requests that follow this (to avoid the problem of
suspending when someone closes the lid after initiating a poweroff).

We also introduce a bit of a dirty hack here: if a suspend is requested
within 1 second of waking up from the previous suspend, ignore it.  This
should help deal with the "double suspend" issues we sometimes see.
Send 'JobRemoved' signal

We should be sending these when starting jobs, but it didn't matter for
timedated.  Now that we are interacting with logind we need to step up
our game a bit.
systemd-shim 3