We are deprecating sysdb->domain so kill the function that gives access to
this member as we should stop relying on it being available (or correct).

To make configuration easier the IPA subdomain provider should be always
loaded if the IPA ID provider is configured and the subdomain provider
is not explicitly disabled. But to avoid the overhead of regular
subdomain requests in setups where no subdomains are used the IPA
subdomain provider should behave differently if configured explicit or
implicit.

If the IPA subdomain provider is configured explicitly, i.e.
'subdomains_provider = ipa' can be found in the domain section of
sssd.conf subdomain request are always send to the server if needed.

If it is configured implicitly and a request to the server fails
with an indication that the server currently does not support subdomains
at all, e.g. is not configured to handle trust relationships, a new
request will be only send to the server after a long timeout or after
a going-online event.

To be able to make this distinction this patch save the configuration
status to the subdomain context.

Fixes https://fedorahosted.org/sssd/ticket/1613

There are case where the extdom extended operation will return the flat
or NetBIOS name of a domain instead of the DNS domain name. If this name
is available for the current domain we accept it as well.

Related to https://fedorahosted.org/sssd/ticket/1561

https://fedorahosted.org/sssd/ticket/1457

https://fedorahosted.org/sssd/ticket/1456

When new subdomains are discovered, the SSSD creates a file that
includes the domain-realm mappings. This file can in turn be included in
the krb5.conf using the includedir directive, such as:

includedir /var/lib/sss/pubconf/realm_mappings