hardware-assisted soft interrupts on all boards.
(Note: VMEChip2-less 162/172 not yet tested)
This greatly simplifies the `rei' path and allows
interrupt nesting to be tracked somewhat more easily.
As a result we now have a working CLKF_INTR() macro
and can detect uvm_fault() being called from an interrupt
(although there may still be a very short race detecting
the latter; need to investigate further).
These changes add support for:
o The MI VMEbus framework on both MVME147 and MVME167.
o Enhancements to the existing MD bus_space(9) implementation.
o Most of the bus_dma(9) API.
scw