terminal/sysview: introduce SETTLE events

terminal/sysview: introduce SETTLE events

This introduces a new SYSVIEW_EVENT_SETTLE notification that is sent after
initial scanning via sysview is done. This is very handy to let the
application raise warnings in case requested resources are not found
during startup.

The SETTLE event is sent after systemd-logind and udev enumerations are
done. This event does in no way guarantee that a given resource is
available. All it does is notify the application that scanning is done!
You must not react to SETTLE if you don't have external synchronization
with the resource you're waiting for.

The main use-case for SETTLE is to run applications _inside_ of logind
sessions and startup sysview. You really want to make sure that the own
session you're running in was found during enumeration. If not, something
is seriously wrong.