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
Approved by: re (cperciva)

(cherry picked from commit 7142ab4790666022a2a3d85910e9cd8e241d9b87)
(cherry picked from commit 0939f3a046533a2df1d2b10fd914c98755dacb0b)