exec family functions take CString instead of &CStr

[canndrew]

For example, the signature of execve is:

pub fn execve(path: &CString, args: &[CString], env: &[CString]) -> Result<Void>

Surely there's no point in taking a &CString as opposed to a &CStr, right? And couldn't the slices just contain &CStrs since it's not possible to move out of a slice anyway?

[asomers]

Yep, there's very rarely a reason for a &CString. Would you care to submit a PR to fix it?