ubus: implement ubus methods to force a Renew and Release

ubus: implement ubus methods to force a Renew and Release

Problem: Renew/Information-Request or Release messages are sent when
odhcp6c receives signal SIGUSR1 or SIGUSR2.
It should also be possible to trigger these messages via a ubus methods.

Solution: Implement two new ubus methods : release() and renew().

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]>