Enhance experimental option rom "threading" - enable preemption.

Enhance experimental option rom "threading" - enable preemption.

When experimental support for parallelizing option roms and hardware
   init (default disabled) is selected, add support for checking on
   hardware init progress from the RTC irq handler.
Enable ability for RTC to be turned on for additional users.
Allow regular option roms (not just vga option roms) to run in
   parallel with hardware init.
Don't use stack in transition32 / transition16 until new mode is
   entered.
Also, cleanup leaking of data handlers in usb code.
Also, decrease frequency of iomemcpy checks (every 2K instead of 1K).