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 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.

The original sysdb code had a strong assumption that only users from one
domain are saved in the databse, with the subdomain feature, we have
changed reality, but have not adjusted all the code arund the sysdb calls
to not rely on the original assumption.

One of the side effects of this incongrunece is that currently group
memberships do not return fully qualified names for subdomain users as they
should.

In oreder to fix this and other potential issues surrounding the violation
of the original assumption, we need to fully qualify subdomain user names.
By savin them fully qualified we do not risk aliasing local users and have
group memberhips or other name based matching code mistake a domain user
with subdomain usr or vice versa.

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.

 * Allows different user/domain qualified names for different
   domains. For example Domain\User or user@domain.
 * The global re_expression and full_name_format options remain
   as defaults for the domains.
 * Subdomains get the re_expression and full_name_format of
   their parent domain.

https://bugzilla.redhat.com/show_bug.cgi?id=811663

New option pwd_expiration_warning is introduced which can be set per
domain and can override the value specified by the original
pam_pwd_expiration_warning.

If the value of expiration warning is set to zero, the filter isn't
apllied at all - if backend server returns the warning, it will be
automatically displayed.

Default value for Kerberos: 7 days
Default value for LDAP: don't apply the filter

Technical note: default value when creating the domain is -1. This is
important so we can distinguish between "no value set" and 0. Without
this possibility it would be impossible to set different values for LDAP
and Kerberos provider.

subdomain_homedir - if set, it contains default value, can be overriden
in further processing