net/netcheck, wgengine/magicsock: make netmon.Monitor required

This has been a TODO for ages. Time to do it.

The goal is to move more network state accessors to netmon.Monitor
where they can be cheaper/cached.

Updates tailscale/corp#10910
Updates tailscale/corp#18960
Updates #7967
Updates #3299

Change-Id: I60fc6508cd2d8d079260bda371fc08b6318bcaf1
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscale/cli/netcheck.go

@@ -53,6 +53,7 @@ func runNetcheck(ctx context.Context, args []string) error {
 		return err
 	}
 	c := &netcheck.Client{
+		NetMon:      netMon,
 		PortMapper:  portmapper.NewClient(logf, netMon, nil, nil, nil),
 		UseDNSCache: false, // always resolve, don't cache
 	}