This commit adds a new type to magicsock, epAddr, which largely ends up
replacing netip.AddrPort in packet I/O paths throughout, enabling
Geneve encapsulation over UDP awareness.
The conn.ReceiveFunc for UDP has been revamped to fix and more clearly
distinguish the different classes of packets we expect to receive: naked
STUN binding messages, naked disco, naked WireGuard, Geneve-encapsulated
disco, and Geneve-encapsulated WireGuard.
Prior to this commit, STUN matching logic in the RX path could swallow
a naked WireGuard packet if the keypair index, which is randomly
generated, happened to overlap with a subset of the STUN magic cookie.
Updates tailscale/corp#27502
Updates tailscale/corp#29326
Signed-off-by: Jordan Whited <jordan@tailscale.com>
66ae8737f4