nic_router: drop fragmented IPv4

The NIC router used to ignore the IPv4 header fields "More fragments" and
"Fragment offset" completely. Therefore higher-level protocols of fragmented
IPv4 were interpreted wrong because each fragment was considered a self-
standing packet, expecting, for instance UDP/TCP headers somewhere inside of
the UDP/TCP data field. Normally, such packets were dropped as soon as the
UDP/TCP checksum check failed because of the misinterpretation. However,
it was also possible for fragmented IPv4 to pass the router although normally
only partially.

IPv4 fragmentation support in the router would introduce some potential
security risks and is presumably not an easy endeavor. So, for now, we settled
on not supporting IPv4 fragmentation. With this commit, the router simply drops
all fragmented IPv4. This is reflected to the log for each fragment as "drop
packet (fragmented IPv4 not supported)" when 'verbose_packet_drop="yes"' is
configured.

The new test 'run/nic_router_ipv4_fragm' is an automated test for this
behavior. The test is added to the autopilot list.

Ref #4236

tool/autopilot.list

@@ -42,6 +42,7 @@ nic_bridge
 nic_bridge_stress
 nic_dump
 nic_router
+nic_router_ipv4_fragm
 nic_router_disable_arp
 nic_router_dhcp_managed
 nic_router_dhcp_unmanaged