net-timestamp: expand documentation and test

Documentation:
  expand explanation of timestamp counter

Test:
  new: flag -I requests and prints PKTINFO
  new: flag -x prints payload (possibly truncated)
  fix: remove pretty print that breaks common flag '-l 1'

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