packets: Define ICMP types.

packets: Define ICMP types.

Linux and FreeBSD have slightly different names for these constants.
Windows doesn't define them.  It is simpler to redefine them from
scratch for OVS.  The new names are different than those used in Linux
and FreeBSD.

These definitions will be used by a future commit.

Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>
Acked-by: Joe Stringer <joe@ovn.org>
Acked-by: Flavio Leitner <fbl@sysclose.org>
Acked-by: Ryan Moats <rmoats@us.ibm.com>