Collapse interrupt thread priorities.

[jhb]

Summary

Allow high priority hardware interrupts to run at PI_REALTIME via
INTR_TYPE_CLK, but collapse all other hardware interrupt threads to
the next priority level (PI_INTR). Collapse all SWI priorities to
the same priority level (PI_SOFT) just below PI_INTR.

Sponsored by: Netflix

[markj]

sys/sys/priority.h

101

IMO the comment above the SWI_* definitions should be updated to note that the distinction is now basically meaningless.

[jhb]

Update document above SWI_* constants.