We will now eliminate any unknown options and providers to
guarantee that the domain is safe for use.

Fixes: #531

This option (applicable to access_provider=ldap) allows the admin
to set an additional LDAP search filter that must match in order
for a user to be granted access to the system.

Common examples for this would be limiting access to users by in a
particular group, for example:
ldap_access_filter = memberOf=cn=access_group,ou=Groups,dc=example,dc=com

This adds two new options:

ipa_dyndns_update: Boolean value to select whether this client
should automatically update its IP address in FreeIPA DNS.

ipa_dyndns_iface: Choose an interface manually to use for
updating dynamic DNS. Default is to use the interface associated
with the LDAP connection to FreeIPA.

This patch supports A and AAAA records. It relies on the presence
of the nsupdate tool from the bind-utils package to perform the
actual update step. The location of this utility is set at build
time, but its availability is determined at runtime (so clients
that do not require dynamic update capability do not need to meet
this dependency).

If the configuration option krb5_store_password_if_offline is set to
true and the backend is offline the plain text user password is stored
and used to request a TGT if the backend becomes online. If available
the Linux kernel key retention service is used.

We had a hard-coded timeout of five seconds for DNS lookups in the
async resolver. This patch adds an option 'dns_resolver_timeout'
to specify this value (Default: 5)

There was a faulty check in get_domain() that led to the
*_provider options being re-added, sometimes after options related
to them had already been set. If those options had a default
value, they would be overwritten by the default.

Fixes: https://fedorahosted.org/sssd/ticket/441