OSX/TimeGlue: Replace deprecated functions, cleanup.

Replace deprecated UpTime() and some time conversion functions by their
mach_absolute_time() et al. counterparts. They return the same values
from the same timebase, just with a bit less hassle.

Throw away TickCount(), hard-code GetSecs time at tickCount zero to
zero. We don’t use this functionality anymore anyway.

-> Mex files compile now without such deprecation warnings, except for
some unrelated warnings in Screen().