Authorization: Negotiate sent to foreign origin

When processing an HTTP redirect to a URL containing userinfo (a
`username:password` component), curl might erroneously retain and re-transmit
the original host's HTTP Negotiate authentication state. Consequently, a
malicious server can trigger a redirect containing arbitrary userinfo to force
curl to send a freshly minted Kerberos ticket—generated using the ambient
identity of the client process to an untrusted third-party host, effectively
bypassing the domain-scoping restrictions normally enforced by the absence of
`--location-trusted`.