Make sigaction() take Option<_> arguments

[ids1024]

It looks like the kernel handles null pointers correctly, but syscall doesn't provide a way to send them.

This is a breaking change, but that shouldn't be an issue since the API is new.

[ids1024]

An alternative would be to make the function take raw pointers, but that would mean it should be unsafe. Although, perhaps registering a signal handler is fairly unsafe anyway.

[jackpot51]

This is good