Similarly to allowing link-local multicast in #13661, we should also allow broadcast traffic
on permitted interfaces when the killswitch is enabled due to exit node usage on Windows.
This always includes internal interfaces, such as Hyper-V/WSL2, and also the LAN when
"Allow local network access" is enabled in the client.
Updates #18504
Signed-off-by: Nick Khyl <nickk@tailscale.com>