use correct data size when sending ICMP Timestamp

use correct data size when sending ICMP Timestamp

ICMP Timestamp messages have a fixed size.  The data portion
following the ICMP header comprises 12 octets to hold three
timestamps of 4 octets each.

Giving the --icmp-timestamp option now automatically sets the
data size to 12 octets.

Error out when -b N is used together with --icmp-timestamp.

Adjust man page to mention incompatibility of -b N, --size=N
with --icmp-timestamp.  Also mention in man page that
cap_net_raw can be used instead of root privileges for ICMP
Timestamp.