`sigaction` should probably be `unsafe`

[tbu-]

If you access thread-local data in a signal handler or panic, you probably invoke UB.

http://users.rust-lang.org/t/unix-signals-in-rust/733/3

[carllerche]

Pinging @wycats

[l0kod]

cc Safe async/reentrant closure

[tbu-]

Meh. It's relatively easy to mark the function unsafe. Should I make a pull request?