{control,net}: close idle connections of custom transports

I noticed a few places with custom http.Transport where we are not
closing idle connections when transport is no longer used.

Updates tailscale/corp#21609

Signed-off-by: Anton Tolchanov <anton@tailscale.com>

control/controlclient/direct.go

@@ -333,6 +333,9 @@ func (c *Direct) Close() error {
 		}
 	}
 	c.noiseClient = nil
+	if tr, ok := c.httpc.Transport.(*http.Transport); ok {
+		tr.CloseIdleConnections()
+	}
 	return nil
 }