Use shm_open instead of mkstemp for anon files

[mstoeckl]

shm_open is more reliable than mkstemp, which can fail if e.g. XDG_RUNTIME_DIR is full, not writeable, or invalid.

(After this is reviewed, I'll make matching PRs for other sway-* projects still using mkstemp.)

Build is currently failing with:

meson build/ --fatal-meson-warnings -Dauto_features=enabled
Traceback (most recent call last):
[...]
    raise PackageNotFoundError(name)
importlib.metadata.PackageNotFoundError: No package metadata was found for meson

[mstoeckl]

Updated again, to restart the check -- and because I had earlier inserted an extra post-decrement, which made retries decrease by 2 for each iteration. That is now fixed.

[emersion]

Thanks!