core: propagate triggered unit in more load states

core: propagate triggered unit in more load states

In 4c2ef3276735ad9f7fccf33f5bdcbe7d8751e7ec we enabled propagating
triggered unit state to the triggering unit for service units in more
load states, so that we don't accidentally stop tracking state
correctly.

Do the same for our other triggering unit states: automounts, paths, and
timers.

Also, make this an assertion rather than a simple test. After all it
should never happen that we get called for half-loaded units or units of
the wrong type. The load routines should already have made this
impossible.