This adds a fallback using `shm_open`/`shm_unlink` for platforms where
`memfd_create` doesn't exist. This seems to be how this is normally
handled, though it's a bit ugly.
This also builds the wayland/x11 code for NetBSD/OpenBSD/DragonFlyBSD.
Add CI builds for FreeBSD and NetBSD. We would need some kind of
virtualisation though to actually run tests on such targets.
I've tested the `shm_open` logic on Linux, but haven't run it on any
BSDs.
