https://fedorahosted.org/sssd/ticket/2009

If the IPA server mode is on and the SSSD is running on the IPA server,
then the server's extdom plugin calls getpwnam_r to read info about trusted
users from the AD server and return them to the clients that called the
extended operation.

The SSSD returns the subdomain users fully-qualified, ie "user@domain"
by default. The format of the fully qualified name is configurable.

However, the extdom plugin returns the user name without the domain
component.

With this patch, when ipa_server_mode is on, warn if the full_name_format
is set to a non-default value. That would prompt the admin to change the
format if he changed it to something exotic.

warning: format string is not a string literal (potentially insecure)
[-Wformat-security]

https://fedorahosted.org/sssd/ticket/1992

Some krb5 functions needn't be available for retrieving ccache
with principal. Therefore ifdef is used to solve this situation with older
version of libkrb5. There were two functions with similar functionality
in krb5_child and krb5_utils. They were merged to one universal function, which
was moved to file src/util/sss_krb5.c

The mpg flag will be read from the cache for subdomains and the domain
object will be created accordingly.

This patch reuses the code from IPA provider to make sure that
domain-realm mappings are written even for AD sub domains.

DIR:/run/user/1000/krb5cc is valid ccname, but function sss_krb5_cc_file_path
returned NULL in this case.