It only works for sending a signal to a process that's in userspace code.
We implement reception by synthesizing a PUSHA+PUSHF in the receiving process
(operating on values in the TSS.)
The TSS CS:EIP is then rerouted to the signal handler and a tiny return
trampoline is constructed in a dedicated region in the receiving process.
Also hacked up /bin/kill to be able to send arbitrary signals (kill -N PID)
