input.conf: switch chapter seek next/previous keys

"UP" seeks forward, so "PGUP" should skip to the next chapter.

Fixes #998.
etc: add mplayer-input.conf
command: run OSD display code even if a property is unavailable

Trying to jump chapters in a gile that has no chapters does nothing,
not even show a warning. This is confusing. The reason is that the
"add chapter" command will just bail out completely if the property
is unavailable.

This was because it exited when it couldn't get the property type.
Instead of exiting, just don't enter the code that needs the type.

(I'm not sure when this behavior changed. I consider it a regression.
It was probably caused by changes to the chapter code, which perhaps
started returning UNAVAILABLE instead of OK if there are no chapters.)
etc/example.conf: update cache options

Some of them changed semantics or got renamed.

Note that the replacements in the example.conf are not necessarily the
equivalents of the replaced options.