player: --save-position-on-quit should always work

Now any action that stops playback of a file (even playlist navigation)
will save the position. Normal EOF is of course excluded from this, as
well as commands that just reload the current file.

The option name is now slightly off, although you could argue what the
word "quit" means.

Fixes #1148 (or at least this is how I understood it).
osd: don't let slow commands cut OSD messages short

Done for screenshot commands, requested by a user.
matroska: look for all known matroska file extensions

For segment linking (this mechanism matches file extensions to avoid
opening files which are most likely not Matroska files in order to speed
up scanning).