We got a complaint here that trashing via the
portal deletes the target of a symlink, not the
symlink itself. It turns out that following the
symlink already happens on the glib side.
https://github.com/flatpak/xdg-desktop-portal/issues/412
Implement the approach suggested in
https://gitlab.gnome.org/GNOME/glib/merge_requests/276
1. Try to open O_RDWR. On success, pass that fd
2. If EACCESS => fail the trash op, we "need" read-write to successfully trash it
3. If EISDIR => re-open the fd with O_PATH, and pass that (which will fail on snap,
but verify the dir for flatpaks)
