demux_mf: fix breakage after commit 4d016a9
command: add "cache" read-only property
osd: show cache state on the playback progression display by default

This affects the "show_progress" command, by defualt on the 'P' key.

If there are complaints, I'll probably remove it again. (It looks
relatively annoying, but it also valueable information... sort of.)
mplayer: print "Cache:" instead of "C:" on the status line

Hasn't been done before because there wasn't enough space, but this
changed recently.
manpage: improve configuration file section a bit