MDL-14883 Password expiration check during login doesn't work after NTLM integration

MDL-14883 Password expiration check during login doesn't work after NTLM integration

Forward ported from MOODLE_19_STABLE.

As part of the NTLM integration work, we moved some login code from
login/index.php to lib/moodlelib.php (function complete_user_login()). This
broke the password expiration check code, as we moved the $userauth variable to
complete_user_login(), and we also need it in login/index.php later.

Add the missing variable and related code to get a suitable password change url.
Also remove the now inexistant $focus variable from the call to print_header().