Currently the user name used in the extdom exop request is
unconditionally set to the short name. While this is correct for the
general name based lookups it breaks UPN/email based lookups where the
name part after the @-sign might not match to domain name. I guess this
was introduce during the sysdb refactoring.

Since sysdb_search_user_by_upn() searches the whole cache we have to set
the domain so that it matches the result.

If Kerberos principals or email address have the same domain suffix as
the domain itself the first user lookup by name might have already added
the name to the negative cache and the second lookup by UPN/email will
skip the domain because of the neg cache entry. To avoid this a special
name with a '@' prefix is used here.