odhcp6c: set server address from Information-request reply

odhcp6c: set server address from Information-request reply

Update the environment variable STATE_SERVER_ADDR with the address
of the DHCPv6 server when a reply to an Information-request message
is received. Without this, there is no way of knowing which server
replied to the Information-request.
This is also consistent with the way that replies to Request and
Rebind messages are handled.

Signed-off-by: Bert Goemans <[email protected]>
Link: https://github.com/openwrt/odhcp6c/pull/83
Signed-off-by: Álvaro Fernández Rojas <[email protected]>