- Add a new ithread_schedule() function to do the bulk of the work of

- Add a new ithread_schedule() function to do the bulk of the work of
  scheduling an interrupt thread to run when needed.  This has the side
  effect of enabling support for entropy gathering from interrupts on
  all architectures.
- Change the software interrupt and x86 and alpha hardware interrupt code
  to use ithread_schedule() for most of their processing when scheduling
  an interrupt to run.
- Remove the pesky Warning message about interrupt threads having entropy
  enabled.  I'm not sure why I put that in there in the first place.
- Add more error checking for parameters and change some cases that
  returned EINVAL to panic on failure instead via KASSERT().
- Instead of doing a documented evil hack of setting the P_NOLOAD flag
  on every interrupt thread whose pri was SWI_CLOCK, set the flag
  explicity for clk_ithd's proc during start_softintr().