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.
Open a packet socket, create a link level header, send packet and
close socket. Adding it to a separate file makes testing of the
DHCP sending much easier, as the test program can supply any socket
to the DHCP client code.
