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