mpegenc: limit the maximum muxrate

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

CC: libav-stable@libav.org
(cherry picked from commit 75bbaf2493a71ee66eaabe3c21fadd84d07888de)
Signed-off-by: Anton Khirnov <anton@khirnov.net>
avconv: fix the muxrate values for -target

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

CC: libav-stable@libav.org
(cherry picked from commit 1688eef25385089026aba55da1885f70a57815ab)
Signed-off-by: Anton Khirnov <anton@khirnov.net>
avconv: fix parsing the AVOptions for -target

CC: libav-stable@libav.org
(cherry picked from commit f5245a9c6206878b892adf3ccbccc9311c202af5)
Signed-off-by: Anton Khirnov <anton@khirnov.net>