dhcp: Handle received DHCP Offer message

      Create a function for handling the full IP, UDP and DHCP packet
and tie it to the main loop. Verify IP and UDP headers and checksum.
Creat a new lease structure with using the values supplied in the
DHCP message. Free the lease structure when client is stopped.

Split out socket handling into a creation and a sending part. As a
result modify the test code.