odhcp6c: implement asynchronous handling for DHCPv6 State

odhcp6c: implement asynchronous handling for DHCPv6 State

Problem:
    The DHCPv6 states were previously handled synchronously, meaning
    that while waiting for a response from the server, no other actions
    could be performed in odhcp6c.

    The process would block inside a loop until a valid response was
    received, limiting responsiveness and overall system performance.

Solution:
    The new approach involves sending the requests and then
    asynchronously monitoring the DHCPv6 socket using poll(). By reading
    a response only when there is data available on the socket, the
    system remains non-blocking.

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]>