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.