This reverts commit f3b97438.

It creates a circular reference holding. (playlist is the parent of the preparser and hold a reference to the preparser during all the preparser life's span).

(Not-So-Dumb-References-Checker IDentifier one)

I saw you, evil preparser thread...

If something depended on this behaviour, it should really be fixed...

Now, we still need to remove the vlc/vlc.h hack.

The signal thread may catch a signal before the interface thread is
initialized. We need to check signum before the first wait.

Also fix a minor timing problem in the screensaver plugin.

It was a misdesign to have it return b_die, due to the race condition
mentioned earlier.