selftests: add txtimestamp kselftest

Run the transmit timestamp tests as part of kselftests.

Add a txtimestamp.sh test script that runs most variants:
ipv4/ipv6, tcp/udp/raw/raw_ipproto/pf_packet, data/nodata,
setsockopt/cmsg. The script runs tests with netem delays.

Refine txtimestamp.c to validate results. Take expected
netem delays as input and compare against real timestamps.

To run without dependencies, add a listener socket to be
able to connect in the case of TCP.

Add the timestamping directory to the kselftests Makefile.
Build all the binaries. Only run verified txtimestamp.sh.

Signed-off-by: Willem de Bruijn <willemb@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>