Draft: util/shm: implement memfd shm support

This merge request adds memfd support for the shm file system. Unsure if its desired which is why it's marked draft for now. I have no decent way of proving this improves performance but it seems to superficially benefit how responsive my system feels (running wayfire and a patched copy of wlroots). This is potentially due to having less overhead having to not go through a full SHM filesystem layer and touching a memory backed file directly?