run sessions and their clients with daemonizable threads

[Antoine Martin]

At the moment, xpra supports this on the server: the xpra server stays alive even when shifter_server is stopped.
Ideally, we want to keep all the sessions running and just re-load the session files when the server re-starts. The problem comes from the fact that NX writes to stdout and we watch this output to get the current session state, we will need to redirect to a file instead and watch that (and maybe also start it with 'nohup'?)

[Antoine Martin]

mostly done for:

• r2598: VNC, NX, dbus
• r2585: RTP audio
• r2634: pulseaudio

We use a wrapper which prints the pid, we then capture it from the server and store it. When reloading the pids, we ensure they are still valid (and not just another process re-using that pid number) by testing open file descriptors and checking for the log file. Not pretty but it works, we should eventually change that to use standard pid files (ala stop-start-daemon) fairly easily too.

You can now kill the server, restart it and the existing sessions will still be there.

[Antoine Martin]

The main (large) changeset: r2625