Fix a bug where calling p.destroy() on the result of calling
start_program(...) would cause a segfault when p went
out of scope due to out_file/err_file being closed twice.
Update doc-comments to reflect the current year and trait
names now being capitalized.
Change the behaviour of core::run::Program.destroy to
forcibly terminate the program (as suggested in issue #5632)
Make destroy() send SIGTERM and add a new method called
force_destroy() that sends SIGKILL - as suggested by 
@thestinger.
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.