config, terminal: move PasswordPromptOutput from config to terminal

First, move this variable in order to avoid circular imports between
config and terminal.  (config already depends on terminal.)

But also, it seems PasswordPromptOutput conceptually fits well in
terminal, as the idea is for it to store the output for the terminal.

lib/terminal/terminal.go

@@ -8,8 +8,6 @@ import (
 	"runtime"
 	"sync"
 
-	"github.com/rclone/rclone/fs/config"
-
 	colorable "github.com/mattn/go-colorable"
 )
 
@@ -70,12 +68,15 @@ const (
 var (
 	// make sure that start is only called once
 	once sync.Once
+
+	// PasswordPromptOutput is output of prompt for password
+	PasswordPromptOutput = os.Stderr
 )
 
 // Start the terminal - must be called before use
 func Start() {
 	once.Do(func() {
-		f := config.PasswordPromptOutput
+		f := PasswordPromptOutput
 		if !IsTerminal(int(f.Fd())) {
 			// If output is not a tty then remove escape codes
 			Out = colorable.NewNonColorable(f)