net-timestamp: expand documentation

Expand Documentation/networking/timestamping.txt with new
interfaces and bytestream timestamping. Also minor
cleanup of the other text.

Import txtimestamp.c test of the new features.

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

diff --cc Documentation/networking/timestamping.txt
Simple merge

diff --cc Documentation/networking/timestamping/Makefile
Simple merge

diff --cc Documentation/networking/timestamping/txtimestamp.c
Simple merge