cmd/tailscaled: add support for running an HTTP proxy

This adds support for tailscaled to be an HTTP proxy server.
It shares the same backend dialing code as the SOCK5 server, but the
client protocol is HTTP (including CONNECT), rather than SOCKS.

Fixes #2289

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

tstest/integration/tailscaled_deps_test_darwin.go

@@ -25,6 +25,7 @@ import (
 	_ "net"
 	_ "net/http"
 	_ "net/http/httptrace"
+	_ "net/http/httputil"
 	_ "net/http/pprof"
 	_ "net/url"
 	_ "os"

tstest/integration/tailscaled_deps_test_freebsd.go

@@ -25,6 +25,7 @@ import (
 	_ "net"
 	_ "net/http"
 	_ "net/http/httptrace"
+	_ "net/http/httputil"
 	_ "net/http/pprof"
 	_ "net/url"
 	_ "os"

tstest/integration/tailscaled_deps_test_linux.go

@@ -25,6 +25,7 @@ import (
 	_ "net"
 	_ "net/http"
 	_ "net/http/httptrace"
+	_ "net/http/httputil"
 	_ "net/http/pprof"
 	_ "net/url"
 	_ "os"

tstest/integration/tailscaled_deps_test_openbsd.go

@@ -25,6 +25,7 @@ import (
 	_ "net"
 	_ "net/http"
 	_ "net/http/httptrace"
+	_ "net/http/httputil"
 	_ "net/http/pprof"
 	_ "net/url"
 	_ "os"

tstest/integration/tailscaled_deps_test_windows.go

@@ -29,6 +29,7 @@ import (
 	_ "net"
 	_ "net/http"
 	_ "net/http/httptrace"
+	_ "net/http/httputil"
 	_ "net/http/pprof"
 	_ "net/url"
 	_ "os"