core: path: Re-enter waiting if target is deactivating

core: path: Re-enter waiting if target is deactivating

Previously, path units would remain in the running state while their
target unit is deactivating. This left a window of time where the target
unit is no longer operational (i.e. it is busy deactivating/cleaning
up/etc) but the path unit would continue to ignore inotify events. In
short: any inotify event that occurs while the target unit deactivates
would be completely lost.

With this commit, the path will go back into a waiting state when the
target unit starts deactivating. This means that any inotify event that
occurs while the target unit deactivates will queue a start job.