Original X11 device demuxer patch from Clemens Fruhwirth

- Build system integration is equivalent to RFC #6 patch
- Same rule applies to ffmpeg.c/allformats.[c|h]
- x11grab.c is from Clemens Fruhwirth except the x11 grab
  structure for libavformat registration is renamed to
  match build system integration.

Originally committed as revision 7293 to svn://svn.ffmpeg.org/ffmpeg/trunk