Simplify icmp conversion
Simplify icmp conversion

pprior code "converts" every icmp type - of which only 3 actually get changed (rest keep same value anyhow!). If also uses a SWITCH {} construct rather than lookup + foreach, which is longer and less efficient.
pfsense/pfsense,NewEraCracker/pfsense,phil-davis/pfsense,brunostein/pfsense,ptorsten/pfsense,NewEraCracker/pfsense,dennypage/pfsense,ptorsten/pfsense,pfsense/pfsense,phil-davis/pfsense,jxmx/pfsense,dennypage/pfsense,NewEraCracker/pfsense,brunostein/pfsense,dennypage/pfsense,NewEraCracker/pfsense,jxmx/pfsense,pfsense/pfsense,NOYB/pfsense,dennypage/pfsense,brunostein/pfsense,phil-davis/pfsense,jxmx/pfsense,phil-davis/pfsense,NOYB/pfsense,NOYB/pfsense,ptorsten/pfsense,pfsense/pfsense