lavf: Use av_gettime_relative

The ones left using av_gettime are NTP timestamps (for RTCP,
which is specified to send the actual current realtime clock
in RTCP SR packets), and the NUT muxer timestamper, which is
documented as using wallclock time.

Signed-off-by: Martin Storsjö <martin@martin.st>
mp3dec: fix reading the Xing tag

The quality scale field is only supposed to be present if the fourth bit
is set. In practice, lame always sets it, but other tools might not.

CC:libav-stable@libav.org
doc: reword the mp3 muxer documentation

Make it more structured.
Merge commit '6df9d9b55d3f56ee7782639a7678eeeaf77f14ea'

* commit '6df9d9b55d3f56ee7782639a7678eeeaf77f14ea':
  lavf: Use av_gettime_relative

Conflicts:
	libavformat/hls.c

See: f78bc96b7c1c74caf99a7139cf81ce1110404965
Merged-by: Michael Niedermayer <michaelni@gmx.at>
Merge commit '99143140dea12363af680d02e23cb42cfe191679'

* commit '99143140dea12363af680d02e23cb42cfe191679':
  mp3dec: fix reading the Xing tag

Conflicts:
	libavformat/mp3dec.c

See: 19ff479f694ddccc1fd1fa00eff94f96ca82671b
Merged-by: Michael Niedermayer <michaelni@gmx.at>
Merge commit 'c63418e0a3afbcdbc59fd65c270d45efafe092ce'

* commit 'c63418e0a3afbcdbc59fd65c270d45efafe092ce':
  doc: reword the mp3 muxer documentation

Merged-by: Michael Niedermayer <michaelni@gmx.at>
Allow decoding of Go2Meeting version 5.