Change order of extensions and read_probe checking. This gives the read_probe

a higher priority than extensions. This gives FFM a chance of working. Note
that some of the other probe functions are bit optimistic, and can be
confused by binary data (such as 0x00 0x00 0x01 0xzz) for some values of zz.

This set of changes makes ffserver work again, and fixes the cannot open
video grab device message.

Originally committed as revision 581 to svn://svn.ffmpeg.org/ffmpeg/trunk