trafgen: add csumicmp for ICMP checksum calculation

Both, the IP and ICMP checksums use the same algorithm from RFC1071.
Thus, it's enough if we give csumip an alias to csumicmp, so that it
can be used in trafgen packet configurations. It's calculated from
the ICMP header + data, with value 0 for this field.

Suggested-by: Jon Schipp <jonschipp@gmail.com>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>