This is entirely optional (i.e. failing in this code is non-fatal) and
only enabled on Linux for now. Additionally, this new behaviour can be
disabled by setting the TS_DEBUG_DISABLE_AF_PACKET environment variable.
Updates #3824
Replaces #5474
Co-authored-by: Andrew Dunham <andrew@du.nham.ca>
Signed-off-by: David Anderson <danderson@tailscale.com>
