ifup, ifreload: rename force parameter to ignore-prio

It makes more sense and also avoid generating unwanted <config> node
in case of ifup|ifreload...
show-config, convert: Introduce <meta-data>

Print out <meta-data><config>... instead of the whole <client-state>
client-state: Move handling of control fields to ifworker
client-state: do not use client_state structure at workers
util: add "yes|no" to ni_parse_boolean()
client-state: Introduce <owner> to <config> node

This field contains the current configuration owner's EUID.
fsm: Move readonly check into update functions
client-state: Config removal on ifdown and failure
ifstatus: Do not consider client-state in ifstatus

Rename status NOT_STARTED to UNCONFIGURED
client-state: Remove state
client-state: Set control flags to ifworkers and their children
xml: Fix functions adding numerical attrs
util: Add ni_var_array_remove() and ni_var_array_remove_at()
xml: Add xml_node_del_attr()
client-state: Turn <meta-data> into config node attrs