logind: rework allocation/freeing of inhibitors

logind: rework allocation/freeing of inhibitors

Let's follow our modern style (i.e. return proper errors, use structure
initialization and _cleanup_).

Most importantly: remove state file and FIFO removal from
inhibitor_free() and let's move it to inhibitor_stop().

This makes sure that state files/FIFOs are not removed when the we
terminate logind, i.e. that they can survive logind restarts.

Fixes: #11825