Make log messages subscriber static

Make log messages subscriber static

In other words, the message callbacks are now shared across all LibVLC
instances. That is a requirement to enable:
 - LibVLC applications to catch messages during LibVLC init/deinit,
 - emitting log messages when no VLC object point is available.

That makes no difference for VLC interfaces, since there is only one
LibVLC instances in a VLC process.