SystemTasks: handle special socket open errors. Refs #2927

- Removed dump cleaning up code: never called
- Unlink socket, if stale
Call PHP exec() to invoke db command. Refs #2927

The Process class is deprecated.
Tweaked Tracker behaviour. Refs #2927

- Don't flash a notification when a detached task is started
- Require user authentication
- Throw a 404 error on socket exceptions
Show failure notifications for synchronous events. Refs #2927

* NethPlatform: added setDetachedProcessCondition() method
  (experimental).

* Process: completed migration of features to NethPlatform; it is only
  an interface adapter to Symfony/Process
Merge branch 'b2927'