Support referrals from Windows Server 2003

Although RFC 6806 Section 7 requires servers to indicate a client
referral in a WRONG_REALM message, Microsoft Windows Server 2003
returns this information in a message with error code
PRINCIPAL_UNKNOWN.  Failure to follow the referral in these messages
prevents referral chasing in Windows Server 2003 forests.  Detect
referral messages of this type by checking for a non-empty
client.realm field in the response, and activate the referral logic in
these cases.

[tlyu@mit.edu: style, comments, and commit message]

ticket: 7856 (new)
target_version: 1.12.2
tags: pullup