https://fedorahosted.org/sssd/ticket/1805

Now that the sysdb context does not contain anymore domain related data
we can simply stop creating faxe sysdb context and just reference the
parent context.

A sysdb contains now multiple domains, but the mpg property is a
property of a specific domain not of the underlying database.

Also remove unused sysdb_search_domuser_by_name()

Also allows us to remove sysdb_subdom_get<pw/gr>nam() wrappers and restore
fqnames proper value in subdomains, by testing for a parent domain being
present or not.

Bring it out of sysdb, which will slowly remove internal dependencies on
domains and instead will always require them to be passed by callers.

These functions allow handling of auth tokens in a completely opaque way,
with clear semantics and accessor fucntions that guarantee consistency,
proper access to data and error conditions.

This is useful for wiping passwords, as it prevents the compiler from
optimizing out a memset to zero before a free()