dbus: add helper methods for multiple start/stop with wait

[pboyd04]

This allows for a single call to stop or start multiple units and wait until all the jobs are complete. This may seem a bit trivial, but I found myself needing these methods in a few places and rather than just wrapping this module with my own thought it might make sense to add to the upstream repo.

[pboyd04]

Test failures are not due to this change.