fusefrontend: xattr: return ENOSYS on unsupported flags

We previously returned EPERM to prevent the kernel from
blacklisting our xattr support once we get an unsupported
flag, but this causes lots of trouble on MacOS:
Cannot save files from GUI apps, see
#229

Returning ENOSYS triggers the dotfiles fallback on MacOS
and fixes the issue.

internal/fusefrontend/xattr.go

@@ -55,7 +55,8 @@ func (fs *FS) SetXAttr(path string, attr string, data []byte, flags int, context
 		return fuse.EPERM
 	}
 	if flags != 0 {
-		return fuse.EPERM
+		// Drop this once https://github.com/pkg/xattr/pull/26 is merged
+		return fuse.ENOSYS
 	}
 	if disallowedXAttrName(attr) {
 		return fuse.EPERM