Also don't fail if the XML validator cannot be loaded.

getNextSibling was buggy (previous patch), and the only use doesn't seem right.

This patch fixes a regression brought by 88ba4526d03285a10e9abd59b9053c9d6927661
 that prevented arrows from being functional.

 - for some reason, this worked on Linux (?!?)
 - for some other reason, there doesn't seem to be 2 different modes for add_file

Close #4004

(manual forward port of 3a31af06dbabd4e21a68e46249fdf3209cd8060a)

As it works fine over http if mp4-files are done with mp4box -tight or similar way,
in most cases mp4-files aren't done like that and avformat works much better in those
cases (don't need to seek so much).

lua_osd: add some parameters to the osd.message function to allow the user to set duration and position.

While the configure checks were platform-agnostic, the implementation
was Windows-specific. Worse, this code now conflicts with our Unicode
filesystem layer, which uses the same function names. In practice, this
code was never used.

On Windows, wcstombs() converts UTF-16 to ANSI, not to UTF-8.

They may be inconsistent on broken multi-processor hardware. But the
system tick count is causing worse problems with power saving features.