core: Add trigger limit for path units

core: Add trigger limit for path units

When conditions fail on a service unit, a path unit can cause
PID 1 to busy loop as it keeps trying to activate the service unit.
To avoid this from happening, add a trigger limit to the path unit,
identical to the trigger limit we have for socket units.

Initially, let's start with a high limit and not make it configurable.
If needed, we can add properties to configure the rate limit similar
to the ones we have for socket units.

(cherry picked from commit aaae822b37aa3ca39aebb516fdc6bef36d730c25)