launch_daemon: Refactor handling of ExternalEventSources.

* Refactor ResolveExternalEvent to actually return the resolved event,
not just whether or not one was found, and then skip resolution
in TriggerExternalEvent and ResetStickyExternalEvent, which now
should only be passed ExternalEvents.

* ExternalEventSources now store destination Events, not Jobs,
following on that refactor.

* The second variant of ResolveExternalEvents is dropped,
and instead the Register/Unregister functions are implemented.

* Trigger and ResetSticky are now done in ExternalEventSource,
which now keeps track of whether it has been sticky-triggered
or not, though it does not use this information yet.

These changes should not affect behavior, they largely constitute
a reorganization (though some TODOs are resolved.)

Change-Id: I46a51cac0edb90e90b154ef9c94791cb7a1aad94
Reviewed-on: https://review.haiku-os.org/c/haiku/+/4509
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

launch_daemon: Added basic logging facility

* The daemon now stores many events in am internal log.
* You can use "launch_roster log" to retrieve it.

launch_daemon: Check requirements before launching a job.

* They were only launched, but we didn't check if we can actually
launch them.

launch_daemon: Fixed missing Event member initialization.

* This fixes targets starting seemingly unconditionally.

launch_daemon: Fixed DemandEvent::ToString().

* Reported "event" instead of "demand".

launch_daemon: Added network_available event & condition.

* Not yet tested.

BLaunchRoster::ResetStickyEvent() added.

* Used to unflag sticky events that already have been triggered once.

launch_daemon: Added volume_mounted event.

* Triggered whenever a volume is mounted (surprise!).
* There is no way to specify which volume you are interested in for now
(if someone knows a good use case, I'd be willing to add that,
though :-)).

launch_daemon: Implemented sticky events, and registration.

* Sticky events are events that keep their signal raised, ie. even if
a job is initialized afterwards, it will still be triggered.
* Consolidated naming for external events.
* Events are now registered once they are actually being used. This
allows them to allocate the resources they need to do their thing.

launch_daemon: launching on demand now actually works.

* Was broken in two ways: if only the shortcut "on_demand" was used,
the event didn't get created at all due to a bug in
Events::AddOnDemand().
* Furthermore, _LaunchJob() always triggered a demand, but it should
only do this when not called from a target.

launch_daemon: GCC4 build fix.

Or is also a keyword.

launch_daemon: Jobs were started before their target.

* A job must not be launched when its target hasn't been launched
yet. This fixes Tracker launching when the mount_server scanned
the initial disk, even though the FirstBootPrompt was showing.
* Jobs are no longer initialized when their target has not been launched
yet. This also means that you cannot talk to a service beforehand in
this case.
* Slight refactoring and clarifying, even added some documentation :-)
* _TriggerJob() is now called _LaunchJob(), and does all the checks for
jobs that _LaunchJobs() does now for targets.

launch_daemon: Added support for external events.

* Other apps can register events, and the launch_daemon can act on them
when they are triggered.

launch_daemon: Fixed job event trigger.

* Always check for events before launching a job unconditionally.
* Correctly check for demand before launching a job.

launch_daemon: Fixed NULL events.

* Also, events now can actually be NULL when there is an empty "on"
field in the settings.
* This fixes a crash, too.

launch_daemon: Outlined basic event mechanism.

launch_daemon: GCC4 build fix.

Or is also a keyword.

launch_daemon: Jobs were started before their target.

* A job must not be launched when its target hasn't been launched
yet. This fixes Tracker launching when the mount_server scanned
the initial disk, even though the FirstBootPrompt was showing.
* Jobs are no longer initialized when their target has not been launched
yet. This also means that you cannot talk to a service beforehand in
this case.
* Slight refactoring and clarifying, even added some documentation :-)
* _TriggerJob() is now called _LaunchJob(), and does all the checks for
jobs that _LaunchJobs() does now for targets.

launch_daemon: Added support for external events.

* Other apps can register events, and the launch_daemon can act on them
when they are triggered.

launch_daemon: Fixed job event trigger.

* Always check for events before launching a job unconditionally.
* Correctly check for demand before launching a job.

launch_daemon: Fixed NULL events.

* Also, events now can actually be NULL when there is an empty "on"
field in the settings.
* This fixes a crash, too.

launch_daemon: Outlined basic event mechanism.