Removed contrib download for win32 as it no longer exists.

Replaces version scripts and export-symbols with a cross-platform
solution for symbol visibility, using dllexport on Windows
and visibility("default") with -fvisibility=hidden added to the
compiler flags on GCC or clang.

We need "!= xno" [0] if we want the default to be *enabled*, otherwise passing
--with-external-libudfread actually ends up disabling it.

[0] https://www.gnu.org/software/autoconf/manual/autoconf-2.71/html_node/External-Software.html

Doesn't make any sense to check for upper bound; all possible values of uint16_t are legal.

Required for direct opening of correct clip.
Opening correct clip reduces latency and incorrect PI/PM/CHAPTER events.

Font can't be null

It doesn't make much sense to call hese functions in paraller,
but doing so might cause some harm ...

Avoid issues if bd_open_???() and bd_get_disc_info() are called at the same time

Avoid removing directories until all Xlets have been stopped.

So that cleanup is not bypassed if shutdown fails.