core: rework how transient unit files and property drop-ins work

core: rework how transient unit files and property drop-ins work

With this change the logic for placing transient unit files and drop-ins
generated via "systemctl set-property" is reworked.

The latter are now placed in the newly introduced "control" unit file
directory. The fomer are now placed in the "transient" unit file directory.

Note that the properties originally set when a transient unit was created will
be written to and stay in the transient unit file directory, while later
changes are done via drop-ins.

This is preparation for a later "systemctl revert" addition, where existing
drop-ins are flushed out, but the original transient definition is restored.