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

Fixes: #531

Also update manpage for min_id/max_id to be more clear about how
it relates to primary GID.

* add forgotten ldap_dns_service option
* sync IPA and LDAP options (ldap_pwd_policy and ldap_tls_cacertdir)
* ldap_uri is no longer mandatory for LDAP provider - the default is to
  use service discovery with no address set now. Ditto for krb5_kdcip
  and ipa_server

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.