vfs: convert vfs options to new style

This also
- move in use options (Opt) from vfsflags to vfscommon
- change os.FileMode to vfscommon.FileMode in parameters
- rework vfscommon.FileMode and add tests

cmd/mountlib/rc.go

@@ -10,7 +10,7 @@ import (
 
 	"github.com/rclone/rclone/fs"
 	"github.com/rclone/rclone/fs/rc"
-	"github.com/rclone/rclone/vfs/vfsflags"
+	"github.com/rclone/rclone/vfs/vfscommon"
 )
 
 var (
@@ -85,7 +85,7 @@ func mountRc(ctx context.Context, in rc.Params) (out rc.Params, err error) {
 		return nil, err
 	}
 
-	vfsOpt := vfsflags.Opt
+	vfsOpt := vfscommon.Opt
 	err = in.GetStructMissingOK("vfsOpt", &vfsOpt)
 	if err != nil {
 		return nil, err