handle user@corp.net username correctly

For a user whose pre-Windows2000 logon name (e.g. CORP\james) is different than their post-Windows2000 logon name (e.g. jimmy@corp.net), freerdp-1.1 was not allowing a logon using the latter as a username.  It turns out that both credSSP and clientInfoPDU expect usernames of the form "n@d" to be passed directly rather than breaking them up into separate user and domain fields.  This was confirmed with mstsc & netmon by examining the data passed from mstsc to the RDP server.

This small patch should be ported to master also.