ubus: implement UBus method to get state data immediately

ubus: implement UBus method to get state data immediately

Problem:
    odhcp6c state data are transmitted when the DHCPv6 state changes.
    If a process subscribes after the start of odhcp6c, then it has to
    wait for the next update event.

Solution:
    Add a new UBus method "get_state" to get the latest state data
    immediately.

Signed-off-by: Nicolas BESNARD <[email protected]>
Signed-off-by: Paul Donald <[email protected]>
Link: https://github.com/openwrt/odhcp6c/pull/106
Signed-off-by: Álvaro Fernández Rojas <[email protected]>