[SSSD] sss_get_domain_name: check for fq name first

[Pavel Březina]

Hi,
my recent patch "sysdb_get_user_attr: use fqn for subdomain users" kinda 
broke authentication (thx ab).

The problem was that sysdb_get_user_attr is often called with fully 
qualified name and we basically ended up with "user@domain@domain" name.

One solution would be to translate the name to fq format in IFP. Other 
would be to change the other places so they use just name without the 
domain part. But since sysdb_get_user_attr now calls sss_get_domain_name 
in the same way as other sysdb functions does I think the best solution 
here is to check the name format in this function and response as 
appropriate.