Do not set the lame quality if the user didn't request it.

This makes FFmpeg's mp3 output more similar to lame's output.
Print a warning if a subtitle demuxer changes utf16 to utf8.

This does not fix anything but gives users a chance to
know that they must not pass -sub_charenc UTF-16 to ffmpeg.

Fixes ticket #4059.
Use v4l2 input format automatically if filename starts with "/dev/video"

Signed-off-by: Carl Eugen Hoyos <cehoyos@ag.or.at>
Mention in the documentation that fieldmatch needs cfr input.

This can be improved in the decimate filter but for the moment
explicitely mentioning that mixed telecined and progressive
content is unsupported is an improvement.

Fixes ticket #3968.
Merge remote-tracking branch 'cehoyos/master'

* cehoyos/master:
  Mention in the documentation that fieldmatch needs cfr input.
  Use v4l2 input format automatically if filename starts with "/dev/video"
  Print a warning if a subtitle demuxer changes utf16 to utf8.
  Do not set the lame quality if the user didn't request it.

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