auto merge of #5761 : Dretch/rust/murder-death-kill, r=thestinger

As proposed in issue #5632.

I added some new stuff to libc - hopefully correctly. I only added a single signal constant (SIGKILL) because adding more seems complicated by differences between platforms - and since it is not required for issue #5632 then I figure that I can use a further pull request to flesh out the SIG* constants more.