mpegenc: limit the maximum muxrate

It is written to the file as a 22-bit value.

CC: libav-stable@libav.org
avconv: fix the muxrate values for -target

The mpegenc private option values are in 50-byte units.

CC: libav-stable@libav.org
avconv: fix parsing the AVOptions for -target

CC: libav-stable@libav.org