video: remove lowres support, cut "too slow" message

Remove support for libavcodec lowres feature. This has been removed in
Libav (the field still exists but has no effect).

Remove the "Possible reasons, problems, workarounds:" portion of "Your
system is too SLOW to play this" message which mentioned lowres. Most
of the other contents were also wrong, obsolete, or otherwise
misleading.
demux_lavf: don't autoselect attached pictures as video

Libavformat exports attached cover pictures from audio files as
"video" tracks. Prevent choosing such tracks as automatically shown
video tracks. Opening a video window to show these for some audio
files while other audio files play in terminal only is questionable,
and these also cause technical problems with some files because they
do not behave like real video tracks.
mp_msg: print messages to stdout, statusline to stderr

Previously, mp_msg() printed text to either stderr or stdout depending
on verbosity level (errors/warnings to stderr, rest to stdout). This
split had no significant benefits that I know of, and made capturing
log output harder as you needed to capture both stdout and stderr.
Print all log output to stdout, statusline to stderr. Treat other
updated terminal output the same as statusline.
input: avoid printing --input=cmdlist/keylist output twice

Use mp_msg() instead of printf() to print the output from
--input=cmdlist and --input=keylist, which as a somewhat hacky side
effect avoids printing the output twice.
Merge remote-tracking branch 'upstream/master'