net/tshttpproxy: new package, support WPAD/PAC proxies on Windows

Updates tailscale/corp#553

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

control/controlclient/direct.go

@@ -36,6 +36,7 @@ import (
 	"tailscale.com/log/logheap"
 	"tailscale.com/net/netns"
 	"tailscale.com/net/tlsdial"
+	"tailscale.com/net/tshttpproxy"
 	"tailscale.com/tailcfg"
 	"tailscale.com/types/logger"
 	"tailscale.com/types/opt"
@@ -147,6 +148,7 @@ func NewDirect(opts Options) (*Direct, error) {
 	if httpc == nil {
 		dialer := netns.NewDialer()
 		tr := http.DefaultTransport.(*http.Transport).Clone()
+		tr.Proxy = tshttpproxy.ProxyFromEnvironment
 		tr.DialContext = dialer.DialContext
 		tr.ForceAttemptHTTP2 = true
 		tr.TLSClientConfig = tlsdial.Config(serverURL.Host, tr.TLSClientConfig)