libdvdcss-1.2.0 patch : CSS title key cache

[Adam Jones]

Hi all.

Here's a preliminary patch to add a CSS title key cache to libdvdcss.
It should apply cleanly to 1.2.0, and seems stable for me under Linux -
no guarantees as to other platforms yet, since I don't have them
available to test on... (NB: Samuel, I've made a few cleanups since the
version I mailed you)

It works as follows:

Set the environment variable DVDCSS_USE_CACHE to "1" (or "y" or "t") to
enable the cache. When an encrypted disc is accessed, the cache
directory $HOME/.dvdcss is created and keys are stored in a plain text
file named by the serial number of the DVD.

Next time the same disc is played, it will find the cache file and prime
the dvdcss->titles list with all the known keys.

Please give it a try and let me know what needs cleaning up, fixing,
etc. (as well as changes to conform to the correct coding style).

Thanks.