demux_lavf: workaround for broken libavformat subtitle seeking

The really funny thing about this commit is that this code is added on
top of another work around. Basically, subtitle seeking in libavformat
is completely broken. To make it useful, we have to add yet another
workaround.

The basic problem is that libavformat's subtitle seeking code always
uses the stream time base, instead of AV_TIME_BASE if stream index -1 is
passed to the avformat_seek_file() function.

Fixes github issue #216. Hopefully this will be fixed in ffmpeg too at
some point.
options: fix --volume option range, add some explanations to manpage

The --volume option accepted values up to 10000, but internally, the
value is always clipped to 0-100 range. What makes this even worse is
that --softvol-max suggests that it extends the range of --volume, which
is not the case. (And passing a volume larger than 100 to --volume
didn't even print a warning.)
Merge branch 'master' into lua_experiment