This CL removes DiffServ and ECN constants assigned by IANA because
for now we don't have an appropriate package to put those constants
as part of API. There were used for the type-of-service field of IPv4
header and the traffic class field of IPv6 header.

Also adds ICMPType for convenience, makes use of internal IANA
protocol number constants instead of syscall's to prevent churning of
package syscall in the near future.

R=dave
CC=golang-dev
https://golang.org/cl/9353045