ldap: Fix user name info lookup

The attributes for first and last name were incorrect for Active Directory
ldap: Fix auto detection of Active Directory

supportedCapabilities is not included by default for the rootDse lookup
funciton in the Net_LDAP2 library. Perhaps Windows Server 2003 level Active
Directory always included it.
ldap: Fix user information lookup for Active Directory

The user@domain syntax is not a correct distinguished name when querying
Active Directory for user information. Instead, the search base should be
queried for an object with (userPrinicipalName=user@domain)