fd-util: make fd_in_set() (and thus close_all_fds()) handle invalidated fds in the array
    
    let's handle gracefully if fds in the specified array are already
    invalidated (i.e. negative). This is handy when putting together arrays
    on the fly.
    
    (cherry picked from commit d11c14a9817f6561a30d96d8faea126a4c811af8)