Header elision muxing support.

This only removes 2 bytes from MP3 and MP2 currently.
Up to 4 could be removed from MP3/MP2 though this might need a 2pass muxer.
Primitive code to remove headers from MPEG-1/2/4 is there too but for the
single file I tried it on (the one in the regression tests), it was a loss
because all video frames were >4096 byte, so that it is disabled ATM.

Originally committed as revision 11936 to svn://svn.ffmpeg.org/ffmpeg/trunk