o Improved y4m_guess_sar() and removed MPEGisms.
o Created y4m_dar_* constants for common display aspect ratios.
lavtools/*:
o Patched up lav2yuv.c, lav_common.*, divxdec.cpp accordingly.
mpeg2enc/readpic.c:
o Patched up accordingly. (And cleansed a little.)
docs/*:
o Corrected lav2yuv.1 accordingly.
o Fixed some typos in mjpegtools.1
