A new ICMPResponder interface is introduced to provide different
implementations of how the ICMP flows should return to the QUIC
connection muxer.
Improves usages of netip.AddrPort to leverage the embedded zone
field for IPv6 addresses.
Closes TUN-8640
A funnel is an abstraction for 1 source to many destinations.
As part of this refactoring, shared logic between Darwin and Linux are moved into icmp_posix