new servertype: type = 4 for user@domain (AD)

In a large active directory environment with multiple subdomains
samaccountname is not sufficent; a username like benj may occur
in more than one subdomain, like benj@sudom1.parentdom and
benj@subdom2.parentdom.

servertype 4 allows a login as benj@subdom1 if your ldap filter
string matches the partial userprincipalname, i.e.

  (&...(userprincipalname=<search>.parentdomain))