sftp,ftp: add http proxy authentication support

This change supports the `http://user:pass@host:port` syntax for the
http_proxy setting.

lib/proxy/http.go

@@ -3,6 +3,7 @@ package proxy
 import (
 	"bufio"
 	"crypto/tls"
+	"encoding/base64"
 	"fmt"
 	"net"
 	"net/http"
@@ -55,7 +56,13 @@ func HTTPConnectDial(network, addr string, proxyURL *url.URL, proxyDialer proxy.
 	}
 
 	// send CONNECT
-	_, err = fmt.Fprintf(conn, "CONNECT %s HTTP/1.1\r\nHost: %s\r\n\r\n", addr, addr)
+	user := proxyURL.User
+	if user != nil {
+		credential := base64.StdEncoding.EncodeToString([]byte(user.String()))
+		_, err = fmt.Fprintf(conn, "CONNECT %s HTTP/1.1\r\nHost: %s\r\nProxy-Authorization: Basic %s\r\n\r\n", addr, addr, credential)
+	} else {
+		_, err = fmt.Fprintf(conn, "CONNECT %s HTTP/1.1\r\nHost: %s\r\n\r\n", addr, addr)
+	}
 	if err != nil {
 		_ = conn.Close()
 		return nil, fmt.Errorf("HTTP CONNECT proxy failed to send CONNECT: %q", err)