Integrates libnl to detect adding routes. When a route is added, the
offline status of all back ends is reset. This patch adds no heuristics
to detect whether back end went offline.

Fixes: #456

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.

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).

This reverts commit 973b7c27c0b294b8b2f120296f64c6a3a36e44b7.

While this patch applied cleanly, it was uncompilable. Reverting
until it can be properly merged.

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).

This is the second attempt to let the PAM client and the PAM responder
exchange their credentials, i.e. uid, gid and pid. Because this approach
does not require any message interchange between the client and the
server the protocol version number is not changed.

On the client side the connection is terminated it the responder is not
run by root. On the server side the effective uid and gid and the pid of
the client are available for future use.

The following additional changes are made by this patch:
- the checks of the ownership and the permissions on the PAM sockets are
  enhanced
- internal error codes are introduced on the client side to generate
  more specific log messages if an error occurs

This reverts commit 5a88e963744e5da453e88b5c36499f04712df097.