[auth] Fix password prompt on Windows

It was wrong to just assume that stdin is "0" everywhere.

Followup to 7514fa2

Fixes #2276

github/config.go

@@ -192,7 +192,7 @@ func (c *Config) scanLine() string {
 }
 
 func getPassword() (string, error) {
-	stdin := 0 // syscall.Stdin
+	stdin := int(syscall.Stdin)
 	initialTermState, err := terminal.GetState(stdin)
 	if err != nil {
 		return "", err