core: add function to tell when job will time out

Things will continue when either the job timeout
or the unit timeout is reached. Add functionality to
access that info.
manager: print ephemeral information about running jobs' timeouts

Produces output like:
[  *** ] (1 of 2) A start job is running for slow.service (33s / 1min 30s)

The first nubmer is the time since job start, the second is the job timeout.
manager: rearm jobs timer

It would fire just once.

Also fix units from sec to usec as appropriate.

Decrease the switching interval to 1/3 s, so that when the time
remaining is displayed with 1s precision, it doesn't jump by 2s every
once in a while. Also, the system is feels noticably faster when the
status changes couple of times per second instead of every few
seconds.
core: simplify check for path absoluteness

Just a microopt.
core: require mounts for the root and working directories

We know that launching a unit will fail if some required directories
haven't been mounted yet. There's no point in starting it just to
have it fail even before it gets a chance to run.