logind: implement maintenance time

logind: implement maintenance time

Update frameworks that work automatically in the background
occasionally need to schedule reboots. Systemd-logind already
provides a nice mechanism to schedule shutdowns, send notfications
and block logins short before the time. Systemd has a framework for
calendar events, so we may conveniently use logind to define a
maintenance time for reboots.

The existing ScheduleShutdown DBus method in logind expects a usec_t
with an absolute time. Passing USEC_INFINITY as magic value now tells
logind to take the time from the configured maintenance time if set.

"shutdown -r" leverages that and uses the maintenance time
automatically if configured. The one minute default is still used if
nothing was specified.

Similarly the new 'auto' setting for the --when parameter of systemctl
uses the maintenance time if configured or a one minute timer like the
shutdown command.