dhcpv6: refactor dhcp_request Function

dhcpv6: refactor dhcp_request Function

Problem:
    The current implementation of the dhcp_request function handles both
    sending and receiving messages within the same function. This makes
    the function a blocking one, as it waits for a valid response before
    proceeding, causing potential delays.

Solution:
    The dhcp_request function is refactored into two functions:
    one for sending DHCP messages and another for receiving them.

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]>