sys/unix: Consolidate signal-handling FFI bindings

Both c.rs and stack_overflow.rs had bindings of libc's signal-handling
routines. It looks like the split dated from rust-lang#16388, when (what is now)
c.rs was in libnative but not libgreen. Nobody is currently using the
c.rs bindings, but they're a bit more accurate in some places.

Move everything to c.rs (since I'll need signal handling in process.rs,
and we should avoid duplication), clean up the bindings, and manually
double-check everything against the relevant system headers (fixing a
few things in the process).

Between the last commit and this one, we can now drop `#![allow(dead_code)]`
from c.rs, which should help avoid binding rot in the future.