util: add anon_file.h for all memfd/temp file usage

Merged Val Packett requested to merge (removed):anon-file into master

Move the Weston os_create_anonymous_file code from egl/wayland into util, add support for Linux memfd and FreeBSD SHM_ANON, use that code in anv/aubinator instead of explicit memfd calls for portability.

Previous discussion: https://patchwork.freedesktop.org/patch/198933/

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