Define a new io.systemd.SysUpdate.Notify Varlink interface with a single
OnCompletedUpdate() method, and after sysupdate successfully installs an
update, invoke that method on every socket linked into
/run/systemd/sysupdate/notify/ via varlink_execute_directory(). This
gives other components a hook to react to applied updates (e.g. recompute
a TPM policy, link a freshly downloaded kernel, refresh extensions).
The notification carries the component name, the installed version and the
list of updated resources (transfer id + on-disk path). Subscribers are
free to ignore the parameters and just treat the call as a trigger.
Setting SYSTEMD_SYSUPDATE_FORCE_NOTIFY=1 forces the notification to be sent
even when no update was applied (in which case no resource list is included),
so follow-up work can be triggered unconditionally.
Fixes: #35988
d36bdc9467
