Decode ICMP embedded flows - show which packet actually triggered an error

Destination Unreachable, Time Exceeded, Redirect, Source Quench, and
Parameter Problem (plus their ICMPv6 equivalents) all carry, after
their own fixed header, as much of the packet that triggered the
error as the network could fit - always at least its IP header plus
the first 8 bytes of payload, exactly enough to recover TCP/UDP port
numbers. That embedded flow is the actual reason an ICMP error shows
up in a capture at all, and wasn't shown before this.

Reuses parse_ipv4()/parse_ipv6() directly on the embedded bytes rather
than a separate parser - it's a genuine (if truncated) IP packet, not
a different format. Two small is_error_type() helpers gate which ICMP
types this is attempted for, since echo/timestamp/Neighbor Discovery
types don't carry an embedded packet at all.

Live-verified with a real traceroute to 8.8.8.8 captured on wlp1s0:
genuine Time Exceeded messages from real intermediate routers - this
machine's own gateway, then real ISP infrastructure several hops out
- all correctly showing the flow that triggered them, matching
traceroute's own hop output.

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