Regression tests for path MTU discovery implementation in the kernel.

The test suite runs on the machine LOCAL, the kernel under test is
running on REMOTE.  On LOCAL a Scapy program is simulating a
connection to REMOTE TCP chargen service.  The source address is a
non existing address on FAKE_NET.  The LOCAL machine acts as a
router between REMOTE and virtual FAKE_NET_ADDR and can create ICMP
packets.

After the three-way handshake REMOTE fills the virtual TCP receive
buffer of FAKE_NET_ADDR with generated chars.  The data is not
acknowledged.  Then LOCAL sends a fragmentation-needed ICMP packet
and expects REMOTE to retransmit the TCP data.  It is checked that
the TCP packet from the REMOTE side has the MTU size that was
announced in the ICMP packet.

The same TCP test is done with IPv6 and a packet too big ICMP6.

An IPv6 UDP packet with 1400 octets payload is sent from FAKE_NET
to REMOTE.  The echo answer triggers an ICMP6 packet too big with
1300 MTU limit from LOCAL.  The response to the next UDP echo packet
has to be fragmented by REMOTE.

After removing the gateway route of the PMTU route on REMOTE, the
first IPv6 UDP echo must have 1400 octets payload.  A single ICMP6
packet too big must change the existing PMTU route so that the next
echo is fragmented.

EXAMPLE

To run this test I use the following configuration files.
You should choose a different set of MAC and IP addresses.

- My local machine where I run the regression test:

/etc/hosts
# to login to qemu with SSH via IPv6 link-local
fe80::725f:caff:fe21:8d70%tap0		q70

/etc/hostname.tap0
lladdr fe:e1:ba:d0:d5:6d up
inet 10.188.70.17 255.255.255.0
inet6 fdd7:e83e:66bc:70:3e97:eff:fea7:9b2
!route add -inet 10.188.219.0/24 127.0.0.1 -blackhole
!route add -inet6 fdd7:e83e:66bc:219::/64 ::1 -blackhole

- My qemu where the kernel under test is running

/etc/hostname.vio0
lladdr 70:5f:ca:21:8d:70
inet 10.188.70.70 255.255.255.0
inet6 fdd7:e83e:66bc:70:725f:caff:fe21:8d70
!route add -inet 10.188.219.0/24 10.188.70.17
!route add -inet6 fdd7:e83e:66bc:219::/64 fdd7:e83e:66bc:70:3e97:eff:fea7:9b2

/etc/inetd.conf
chargen stream  tcp     nowait  root    internal
chargen stream  tcp6    nowait  root    internal
echo            dgram   udp6    wait    root    internal

/etc/rc.conf.local
inetd_flags=
sshd_flags=

- My environment when executing the test

LOCAL_IF=tap0
LOCAL_MAC=fe:e1:ba:d0:d5:6d
REMOTE_MAC=70:5f:ca:21:8d:70
REMOTE_SSH=q70

LOCAL_ADDR=10.188.70.17
REMOTE_ADDR=10.188.70.70
FAKE_NET=10.188.219.0/24
FAKE_NET_ADDR=10.188.219.188

LOCAL_ADDR6=fdd7:e83e:66bc:70:3e97:eff:fea7:9b2
REMOTE_ADDR6=fdd7:e83e:66bc:70:725f:caff:fe21:8d70
FAKE_NET6=fdd7:e83e:66bc:219::/64
FAKE_NET_ADDR6=fdd7:e83e:66bc:219::188

- Fix your configuration until make check-setup passes