Remove needless pass by value

See https://rust-lang-nursery.github.io/rust-clippy/v0.0.212/index.html#needless_pass_by_value
Remove unnecessary muts
Add module file for signal testing helper
Don't pin exact libc
Add disclaimer to readme
Minor doc updates and bump Cargo.lock
Make sure repo URL is clone-able
Fix broken keyword in Cargo.toml
Update Cargo.toml and Docs
Add BSD 2 clause license
Format and remove feature gate for stable feature

Also update lock file
Typo fix in the README
Make handle_signal more idiomatic (I think?)
Retab command creation
Simplify arg matching logic
Add TODO to fix unused branch
Ensure child exit status is being set
Exit on interrupts in test program
Make debugln! actually print a newline
Mark a future TODO (Does WIFSIGNALED not exist?)