Ignore DHCP packet sent from non-68 client port

This differs from previous behavior where dnsmasq would reply to port 68
if the client had no configured address (ciaddr empty in request), or
send replies to the client port if the request ciaddr matched the
assigned lease.
Not all DHCP servers preserve this behavior, and there is no good known
use-case for it. Not replying to such packets is less error-prone and
closer to the standard.

Bug: b/109584964
Test: Added test in DhcpServerTest.py passes
Change-Id: I88d467336cc4f4e4c9498c3787ec22fdef5e1cdd