Implement an upcall mechanism to support userland LWKT. This mechanism will

Implement an upcall mechanism to support userland LWKT.  This mechanism will
allow multiple processes sharing the same VM space (aka clone/threading)
to send each other what are basically IPIs.

Two new system calls have been added, upc_register() and upc_control().
Documentation is forthcoming.  The upcalls are nicely abstracted and a
program can register as many as it wants up to the kernel limit (which
is 32 at the moment).

The upcalls will be used for passing asynch data from kernel to userland,
such as asynch syscall message replies, for thread preemption timing,
software interrupts, IPIs between virtual cpus (e.g. between the processes
that are sharing the single VM space).