pf: split ICMP/ICMPv6 number space in pf_icmp_mapping()

In pf_icmp_mapping() the ICMP and ICMPv6 types shared the same
number space. In fact they are independent and must be handled
separately. Fix traceroute via pf by splitting pf_icmp_mapping()
into IPv4 and IPv6 sections.
ok henning@ mcbride@; tested mcbride@; sure deraadt@

MFC after: 1 day
Obtained From: OpenBSD, bluhm <bluhm@openbsd.org> ef4bccd7509e
Sponsored by: Rubicon Communications, LLC ("Netgate")