ubus: emit UBus event on DHCP state changes

ubus: emit UBus event on DHCP state changes

Problem:
    Whenever the DHCPv6 state changes a custom shell script provided in
    an argument on process start is called.

Solution:
    In addition to the current script mechanism, emit a UBus event
    whenever the DHCPv6 state changes.

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]>