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

vfs/dir_test.go

@@ -43,7 +43,7 @@ func TestDirMethods(t *testing.T) {
 	assert.Equal(t, false, dir.IsFile())
 
 	// Mode
-	assert.Equal(t, vfs.Opt.DirPerms, dir.Mode())
+	assert.Equal(t, os.FileMode(vfs.Opt.DirPerms), dir.Mode())
 
 	// Name
 	assert.Equal(t, "dir", dir.Name())