Don't build mplayer and vlc player backends

libvlc probably isn't installed, and it's a waste of time.
VLC backend: improved error reporting

Print the human-readable error and the error code instead of an
unprintable C error code.
Let the Makefile rely on $GOPATH
better log formatting; improved event handling in VLC
Use global initial volume, and set it to 80%
Improve position handling: only update when possible and needed
Remove select with only a single case
VLC player: seek to position if needed when starting playback
(minor) comment change (race condition warning)
do not restart playback when already buffering

When starting playback using the &v= parameter, setPlaylist is also
called. This commit ensures playback is not restarted when the video
hasn't changed, the video is buffering, and the position also hasn't
changed. It does update the playlist, though, if needed.
go fmt
switch player backend to VLC

the VLC backend still doesn't work very well, but it does support
seeking right now as opposed to MPV.
Use volume delta instead of volume value
Prevent extra GetPlaystate() inside setPlayState
Improve concurrent message handling

Added more goroutines, and removed one that was unnecessary
Fall back to SetVolume when there's no delta
bugfix: sometimes, player would be stuck in 'buffering' state

.. while sound was already playing
Improved logging facility

mp.log() ensures all logging is the same
Make sure delta volume changes are within the 0-100 range

Going outside that range could crash media players
Do not skip multiple of the same events

This caused some bug in the libvlc wrapper...