1) Crack the title key (the method that was here before). The only change
here is that we search the key for the exact chapter we are seeking with
DVDSetArea (in case the key has changed within a title). It is maybe not a
good idea.

2) Crack the disc key, which allows us to decode instantly all title keys.
I've used an algorithm from Frank Stevenson ; it eats much memory (64MB),
and takes about 15 s at launch time.

3) Decode the disc key with player keys (libcss method). However, you need
licensed player keys at build time for that to work.

To choose between libdvdcss methods, a command line options is supplied:

        vlc --dvdcss <method> where method is one of title, disc, key.

Note that all these changes only work with linux now, since we have to add a
specific ioctl to read title key. I hope that I haven't broken too many things.