Signal should support arbitrary signals

[joshtriplett]

The Signal enum is missing the real-time signals, from SIGRTMIN to SIGRTMAX.

Handling these will be interesting, because glibc reserves the first two of them, and does so by changing its exposed value of SIGRTMIN.

I would suggest exposing the same SIGRTMIN that glibc does (34 rather than 32), and additionally exposing enum values for the two reserved signal numbers, with names that clearly indicate that they're reserved by libc.

That way, when people port from C to Rust, they won't get surprised by the difference.