This avoids unintentionally adding a newline character at the end of
user passwords when they are set using passwd(1).
I also fixed these two issues:
- The return value of getline() was being saved in an `int` variable
instead of in a `ssize_t` variable; I replaced the `int` keyword with
`auto` to fix this issue.
- Prior to this patch, get_password() could potentially return
tcsetattr()'s errno instead of getline()'s errno in case of an error.
We now make sure it always returns the right errno in case of an error.
