core: add a separate unit directory for transient units

core: add a separate unit directory for transient units

Previously, transient units were created below the normal runtime directory
/run/systemd/system. With this change they are created in a special transient
directory /run/systemd/transient, which only contains data for transient units.

This clarifies the life-cycle of transient units, and makes clear they are
distinct from user-provided runtime units. In particular, users may now
extend transient units via /run/systemd/system, without systemd interfering
with the life-cycle of these files.

This change also adds code so that when a transient unit exits only the
drop-ins in this new directory are removed, but nothing else.

Fixes: #2139