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
Merge commit '75bbaf2493a71ee66eaabe3c21fadd84d07888de'

* commit '75bbaf2493a71ee66eaabe3c21fadd84d07888de':
  mpegenc: limit the maximum muxrate

Conflicts:
	libavformat/mpegenc.c

See: 28d35dbe9f570025c220c08997bc11e21b3c1a66
Merged-by: Michael Niedermayer <michaelni@gmx.at>
Merge commit '1688eef25385089026aba55da1885f70a57815ab'

* commit '1688eef25385089026aba55da1885f70a57815ab':
  avconv: fix the muxrate values for -target

Conflicts:
	ffmpeg_opt.c

No change, as ffmpegs muxrate is in bits/sec

Merged-by: Michael Niedermayer <michaelni@gmx.at>
Merge commit 'f5245a9c6206878b892adf3ccbccc9311c202af5'

* commit 'f5245a9c6206878b892adf3ccbccc9311c202af5':
  avconv: fix parsing the AVOptions for -target

Merged-by: Michael Niedermayer <michaelni@gmx.at>
ffmpeg_opt: reduce diff to libav in opt_target() by 16 lines

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>