ctdb-scripts: Make 10.interface IPv6-safe

ctdb-scripts: Make 10.interface IPv6-safe

Add checking to "releaseip" and "updateip" to ensure that the given IP
address is really on the given interface with the given netmask.  If
reality doesn't match the given arguments then believe reality.

Use new function iptables_wrapper() instead of calling iptables()
directly.

Use new function flush_route_cache() instead of doing IPv4-specific
/proc magic.

Remove setting of otherwise unused variable "failed".

Fix a test for which the error message has changed.

Signed-off-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Amitay Isaacs <amitay@gmail.com>