icmp: do not store per-VNET identical array of strings

We need per-VNET struct counter_rate, but we don't need per-VNET set of
const char *. Also, identical word "response" can go into the format
string instead of being stored 7 times.

Reviewed by: kp, zlei, tuexen
Differential Revision: https://reviews.freebsd.org/D44475