wgengine/monitor: on unsupported platforms, use a polling implementation

Not great, but lets people working on new ports get going more quickly
without having to do everything up front.

As the link monitor is getting used more, I felt bad having a useless
implementation.

Updates #815
Updates #1427

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

wgengine/monitor/monitor_freebsd.go

@@ -25,7 +25,7 @@ type devdConn struct {
 	conn net.Conn
 }
 
-func newOSMon(logf logger.Logf) (osMon, error) {
+func newOSMon(logf logger.Logf, _ *Mon) (osMon, error) {
 	conn, err := net.Dial("unixpacket", "/var/run/devd.seqpacket.pipe")
 	if err != nil {
 		return nil, fmt.Errorf("devd dial error: %v", err)