player: don't show the path part for external subtitle files

Show the filename only. Feature request on IRC.
manpage: explain how to replace --slave-broken
Improve setting AVOptions

Use OPT_KEYVALUELIST() for all places where AVOptions are directly set
from mpv command line options. This allows escaping values, better
diagnostics (also no more "pal"), and somehow reduces code size.

Remove the old crappy option parser (av_opts.c).