Ignore DHCP discover and request w/ invalid giaddr

This matches previous behavior, and there could be situations where
another server on the network would be configured to reply to such
requests, so not replying is better than sending NAKs as done now.

Also refactoring requests in DhcpLeaseRepositoryTest and replacing some
INETADDR_UNSPEC usages for giaddr with INET4_ANY (giaddr is a BOOTP
field and can't be unspecified, only empty).

Test: Following DhcpServerTest.py regression tests pass:
      test_request_selecting_giaddr_outside_subnet
      test_discover_requestaddress_giaddr_outside_subnet
      test_discover_knownaddress_giaddr_outside_subnet
      test_discover_giaddr_outside_subnet
      Also: atest FrameworksNetTests passes
Change-Id: I4decffccfc64d5e0e29c9ce1cf1446644fcf8190