ganeti.daemon: Make Mainloop more flexible

While working on the HTTP client class, I wanted to use Mainloop
before deciding to use threads instead.

- Add docstrings
- Rename "quit" to "running", move it to a local variable
- Support adding and removing I/O listeners while running
  (e.g. to add/remove listeners from OnIO(…) calls)
- Allow user of the class to disable signal handlers
- Flag to exit once all listeners are gone

Reviewed-by: iustinp