demux: rewrite probing and demuxer initialization

Get rid of the strange and messy reliance on DEMUXER_TYPE_ constants.
Instead of having two open functions for the demuxer callbacks (which
somehow are both optional, but you can also decide to implement both...),
just have one function. This function takes a parameter that tells the
demuxer how strictly it should check for the file headers. This is a
nice simplification and allows more flexibility.

Remove the file extension code. This literally did nothing (anymore).

Change demux_lavf so that we check our other builtin demuxers first
before libavformat tries to guess by file extension.

stream/stream_mf.c

@@ -32,7 +32,7 @@ static int
 mf_stream_open (stream_t *stream, int mode, void *opts)
 {
   stream->type = STREAMTYPE_MF;
-  stream->demuxer = "lavf";
+  stream->demuxer = "mf";
 
   return STREAM_OK;
 }