change ICMPv4 String() method to be similar to ICMPv6
change ICMPv4 String() method to be similar to ICMPv6

* introducing constants for ICMPv4 codes (for types constants already exist)

skyportsystems/gopacket,xuzhaokui/gopacket,skyportsystems/gopacket,google/gopacket,levigross/gopacket,adriantam/gopacket,vtolstov/gopacket,adriantam/gopacket,LrsK/gopacket,vtolstov/gopacket,LrsK/gopacket,LrsK/gopacket,google/gopacket,google/gopacket,levigross/gopacket,rouzier/gopacket,ross-williams/gopacket,xuzhaokui/gopacket,rouzier/gopacket,levigross/gopacket,xuzhaokui/gopacket,vtolstov/gopacket,skyportsystems/gopacket,ross-williams/gopacket,rouzier/gopacket,ross-williams/gopacket,adriantam/gopacket