Allow tracing interrupts by defering the wakeup(9) to a different context.

Allow tracing interrupts by defering the wakeup(9) to a different context.

It is currently not safe to call wakeup(9) in interrupt handlers at a priority
higher than IPL_SCHED.  As long as dt(4) relies on generic kernel primitives
we have to play tricks to be able to inspect more parts of the kernel.  In this
case defer the wakeup(9) to a custom soft-interrupt.  This will be good enough
as long as we don't add tracepoints to the soft-interrupt machinery.  A more
complex & viable solution would be to not rely on the kernel generic IPC to
avoid recursion.

From visa@ and Christian Ludwig, ok claudio@