Use mem::zeroed instead of mem::uninitialized to create libc::sigaction and others.

Hello! As part of rust-lang/rust#99389 I'm going through crates that run into stricter checks that are being added, and this crate runs into them.

In signal.rs, the uninitialized::<libc::sigaction>() should use mem::zeroed or MaybeUninit, or directly constructing the struct. Not sure what you want to do, but uninitialized is UB.

Anywhere else that uninitialized is being used is also UB, but might not panic just yet, but might in the future.