lib/random: unify random string generation into random.String

This was factored from fstest as we were including the testing
enviroment into the main binary because of it.

This was causing opening the browser to fail because of 8243ff8bc8.

cmd/cmd.go

@@ -9,6 +9,7 @@ package cmd
 import (
 	"fmt"
 	"log"
+	"math/rand"
 	"os"
 	"os/exec"
 	"path"
@@ -492,6 +493,7 @@ func AddBackendFlags() {
 
 // Main runs rclone interpreting flags and commands out of os.Args
 func Main() {
+	rand.Seed(time.Now().Unix())
 	setupRootCommand(Root)
 	AddBackendFlags()
 	if err := Root.Execute(); err != nil {