Make icmp_print() accept the length variable, which is the length of the

Make icmp_print() accept the length variable, which is the length of the
packet without the IP header.  This is needed by the next commit that
will allow tcpdump to detect bad ICMP checksums.

Related functions like {tcp,udp,icmp6}_print() already accept this
length variable, so this change makes icmp_print() consistent with
them as well.

This commit makes no functional change to tcpdump itself.

OK florian@