xml: xml_document_free() return if NULL
xml: Improve xml_node_is_empty()
utils: ni_file_read() set lenp is present
config: create "nanny" dir in statedir
nanny: Add saving new policies
policy: Use xml_node_is_empty() to check valid node
nanny: Introduce ni_nanny_policy_drop()

Policy is being dropped on:
- disable() method call
- DEVICE_DELETE event
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: Introduce ni_nanny_policy_load()
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.