build: install input config files into doc dir

Users using binary packages don't have obvious access to them, so this
seems like a good idea.
command: don't display non-sense if volume is unavailable

Showed "Volume: (unavailable)%". That was dumb.

The message string is now a bit convoluted; mostly because the property
expand syntax can't do "if-else", just "if".

CC: @mpv-player/stable
gl_x11: distinguish missing GLX and too old GLX versions

This probably led to confusing output.

CC: @mpv-player/stable
af_lavcac3enc: fix byte order

Oops.

Fixes #1172.

CC: @mpv-player/stable
Release 0.6.1