homed: explicitly deactivate all home directories on shutdown

homed: explicitly deactivate all home directories on shutdown

Let's explicitly deactivate all home dirs on shutdown, in order to
properly synchronizing unmounting and avoiding blocking devices.

Previously, we'd rely on automatic deactivation when home directories
become unused. However, that scheme is asynchronous, and ongoing
deactviations might conflicts with attempts to unmount /home. Let's fix
that by providing an explicit service systemd-homed-activate.service
whose only job is to have a ExecStop= line that explicitly deactivates
all home directories on shutdown. This service can the be ordered after
home.mount and similar, ensuring that we'll first deactivate all homes
before deactivating /home itself during shutdown.

This is kept separate from systemd-homed.service so that it is possible
to restart systemd-homed.service without deactivating all home
directories.

Fixes: #16842