seL4: remove dead code variable totwork

Another artifact from the original hw implementation

seL4: Add way to register software event handlers

This makes it possible to register event handlers that will run attached
to a rumpkernel cpu context. This can be used to register event handlers
in custom rump kernel drivers that can receive notification messages
from other seL4 threads, not just hardware interrupts.

seL4: Remove interrupt routing logic

This is not used and is left over from the initial hw implementation

Don't ack msi interrupts

seL4: Add CAmkES support

seL4: Reduce error output boilerplate

seL4: Add seL4 as a target rumprun platform