Add SIGRTMIN and SIGRTMAX constants

[kacejot]

Or at least remove Error on Signal::from_c_int, if signal is not between 0 and NSIG, so I can pass custom signals. Option here is also excess.

[kacejot]

I use transmute for my purpose now and it works fine, but I wish getting rid of unsafe block.

let signal: Signal = unsafe { std::mem::transmute(signal::SIGRTMIN() + 1) };
match kill(pid, signal) {
    ...
}

[asomers]

I think this would be a reasonable change. However, you'll have to add support to libc first. And you'll need to deal with the fact that SIGRTMIN is a constant on some platforms and a function on others.

[mkroening]

FYI, SIGRTMIN and SIGRTMAX have been added to libc in rust-lang/libc#2304.

[mkroening]

This is related to #495.