Added preliminary support for context switches (triggers the interrupts, but hangs after the switch ; something's not quite right yet).
Removed the PFIFO_REINIT ioctl. I hope it's that a good idea...
Requires the upcoming commit to the DDX.
- Mark the correct RAMIN slots as free (oops)
- Remove a VRAM alloc that shouldn't have been there (oops)
- Move HT init out of firstopen() and into dma_init()
- Setup PFIFO_RAM{HT,FC,RO} in pfifo_init()