fsm: export ni_ifworker_fail() and ni_ifworker_success()
fsm: Add ni_ifworker_is_valid_state()
ifdown: Disable devices and stop all policies
nanny: Turn errors to debug

Display only errors from client part. Internal nanny functions should be
lowered to debug.
client: Add policy match generation functions
policy: Export policy checking functions
client: Accept policy validation by default
xml: Add xml_node_is_empty()
Merge pull request #250 from pwieczorkiewicz/nanny_work2