https://fedorahosted.org/sssd/ticket/1495

Currently the only type of supported sub-domains are AD domains which
are not case-sensitive. To make it easier for Windows user we make
sub-domains case-insensitive as well which allows to write the username
in any case at the login prompt.

If support for other types of sub-domains is added it might be necessary
to set the case-sensitive flag based on the domain type.

Domains may have a flat or short name to save some keystrokes when
typing fully qualified user names. Internally sssd will always use the
canonical name to allow consistent processing.

The Active Directory KDC handles request case in-sensitive and it might
not always to possible to guess the UPN with the correct case. We check
if the returned principal has a different case then the one used in the
request and updates the principal if needed. This will help using calls
from the Kerberos client libraries later on which would otherwise fail
because the principal is handled case sensitive by those libraries.

With the current approach the upn was either a pointer to a const string
in a ldb_message or a string created with the help of talloc. This new
function always makes it a talloc'ed value.

Additionally krb5_get_simple_upn() is enhanced to handle sub-domains as
well.

In general Kerberos is case sensitive but the KDC of Active Directory
typically handles request case in-sensitive. In the case where we guess
a user principal by combining the user name and the realm and are not
sure about the cases of the letters used in the user name we might get a
valid ticket from the AD KDC but are not able to access it with the
Kerberos client library because we assume a wrong case.

The client principal in the returned credentials will always have the
right cases. To be able to update the cache user principal name the
krb5_child will return the principal for further processing.