This is a fallout from my work on the HTTP client class. Repeating
timers are prepared, but not yet implemented.

Reviewed-by: iustinp

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

We don't have timers yet, hence waking up is not needed at all.

Reviewed-by: ultrotter

This fixes removals of the pid files and clean exit in the init script

Reviewed-by: iustinp

There is no point in initializing sigchld_handler as an empty dict, when
it will be overwritten by an instance of utils.SignalHandler
immediately after.

Reviewed-by: iustinp

This mainloop can be used in daemons like ganeti-noded.

Reviewed-by: iustinp