Implement daemon mode for Nuxeo Drive under POSIX (ndrive start / stop)

Launching the synchronization process should be possible using:

ndrive start

The synchronization process should check if another existing running synchronization process is already running and stop if it's already the case.

It should be possible to stop the running synchronizer process with:

ndrive stop

Note: under windows, the binary freezer / packager already generates a ndrivew.exe binary that starts in detached mode hence it is not necessary to daemonize this process in the traditional posix way: ndrive start can 'just' call subprocess.Popen("ndrivew.exe") and exit.