- Implement support for the asynchronous IPI calls.
- Rework synchronous IPI code to reuse the asynchronous mechanism.
- Add ipi(9) manual page; needs wizd(8).
Note: MD code can now provide a low level primitive for the ipi(9) and
reuse this interface instead of open-coding. Portmasters are encouraged
to convert. Ride 6.99.43!