nanny: Refactor ni_objectmodel_nanny_create_policy()

- Accepts only single, non-empty policy node
policy: remove const from ni_fsm_policy_transform_document()
nanny: create and update workers for existing policy
nanny: Introduce ni_nanny_create_policy()
nanny: create policy from one place only

Incorporate ni_nanny_create_policy() into
ni_objectmodel_nanny_create_policy()
fsm, nanny: proper check for workers config presence
nanny: Do not recheck pending devices

If device is not present, recheck only factory devices.
Ignore pending and non-existing physical devices.
ifreload: add nanny variant
ifreload: Perform ifup part only if config exists
xml: xml_document_free() return if NULL
xml: Improve xml_node_is_empty()
utils: ni_file_read() set lenp is present
policy: Use xml_node_is_empty() to check valid node
Merge pull request #346 from pwieczorkiewicz/ifreload-nanny

nanny, client: fixes for updating workers if policy exists and Ifreload nanny variant
Merge pull request #347 from pwieczorkiewicz/small-fixes

couple of nanny related small fixes
dhcp4: wait and discover if unable to rebind/reboot.
Merge pull request #348 from mtomaschewski/dhcp4-reboot

dhcp4: wait and discover if unable to rebind/reboot.
ifreload: fire nanny on down, status info cleanup
Merge pull request #349 from mtomaschewski/cleanup

ifreload: fire nanny on down, status info cleanup
version 0.5.38