GME starts all track indexes at zero, but subtune prefixes
start at one. This fixes an off-by-one error during track
enumeration.

If you load an NSFE file (which has embedded track titles),
then attempt to load an M3U file, it causes GME to lose all
information found in the NSFE file. This adds a check that
the M3U file exists before attempting to load.

Enforcing MADV_DONTFORK is a surprising limitation for this library
which aims to be generic.

Our IcuCaseFold() fallback using strxfrm() is not actually case
insensitive.  This commit fixes the problem by switching to
strcasecmp().  That function is not guaranteed to support UTF-8, but
it's the best we can do in this sparse situation.

Closes #111

RoarAudio's sndio emulation has been a source for annoyances.  First,
their headers turned out to be broken with C++, due to their use of
the "new" keyword.  Then they used a preprocessor macro to rename
"sio_hdl" to something else, effectively disallowing the use of
forward declarations.  Enough is enough, and I'm removing support for
it.

RoarAudio users should better use the RoarAudio output plugin.