For example, if an exec command is running when you Ctrl-C the Pebble
daemon, it hangs.

It's hanging because the ServiceManager is stopping the reaper early in
the process, before TaskRunner has had a chance to abort the exec tasks
(aborting an exec task sends SIGKILL to its pid via the tomb and
command context).

Then when TaskRunner.Stop is called, it calls tomb.Kill and then
tomb.Wait on each task (exec) tomb, and because the reaper's not
running, the tomb.Wait hangs.

So the fix is to move the reaper.Start and reaper.Stop to the top
level (inside the "run" command, which is also used for "enter"),
instead of putting them in servstate.NewManager and
ServiceManager.Stop.

After doing this, some of the tests also had to be modified to start
and stop the reaper.

Fixes canonical#163