Implement mechanism to deliver hardware interrupts to userspace processes by

Implement mechanism to deliver hardware interrupts to userspace processes by
co-opting the sigaction() syscall in combination with a new signal, SIGHWINT.
The signal mask that is passed to sigaction() by means of a struct sigaction
is used to notify the kernel which interrupts the process would like to receive.
Processes that choose to use the same handler for multiple hardware interrupts
should set the SA_SIGINFO flag in the sigaction structure. The interrupt handler
will then be passed an additional siginfo_t structure (and a struct ucontext*)
which can be used to find out the hardware interrupt number by inspecting the
siginfo_t structure's si_trapno member.