Netcheck no longer performs I/O itself, instead it makes requests via
SendPacket and expects users to route reply traffic to
ReceiveSTUNPacket.
Netcheck gains a Standalone function that stands up sockets and
goroutines to implement I/O when used in a standalone fashion.
Magicsock now unconditionally routes STUN traffic to the netcheck.Client
that it hosts, and plumbs the send packet sink.
The CLI is updated to make use of the Standalone mode.
Fixes#8723
Signed-off-by: James Tucker <james@tailscale.com>
