mount,mount2,cmount: fix --allow-non-empty #3562

Since version 3 of fuse libfuse no longer does anything when given the
nonempty option and it's default is to allow mounting over non empty
directories like normal mount does.

Some versions of libfuse give an error when using `--allow-non-empty`
which is annoying for the user.

We now do this check ourselves so we no longer need to pass the option
to libfuse.

Fixes #3562

cmd/mount/mount.go

@@ -34,9 +34,6 @@ func mountOptions(VFS *vfs.VFS, device string, opt *mountlib.Options) (options [
 	if opt.AsyncRead {
 		options = append(options, fuse.AsyncRead())
 	}
-	if opt.AllowNonEmpty {
-		options = append(options, fuse.AllowNonEmptyMount())
-	}
 	if opt.AllowOther {
 		options = append(options, fuse.AllowOther())
 	}