icmp: split icmp type printing to header file

libebt_ip will get icmp support soon, unify icmp name
mapping a bit so we can re-use this code from libebt_ip.c.

It doesn't seem to be too useful to move to libxtables
(as its icmp specific), so add a hedaer file for this to reduce
copy & paste needs.

Signed-off-by: Florian Westphal <fw@strlen.de>
3 files changed