Apparently when I rebased https://github.com/flatpak/flatpak/pull/1314
to master, git cleanly applied the patch in a different place than it
was originally. Commit c5ab9e22b moved the find_remotes_async() call to
find_latest_rev(), so all that needs to be done is pass along the
checksum in check_for_update(). This way when you're trying to downgrade
something the specified commit will be searched for instead of the
latest one.

Fixes https://github.com/flatpak/flatpak/issues/1431

flatpak_context_append_bwrap_filesystem() didn't actually return the
exports so callers crashed dereferencing NULL.

This fixes https://github.com/flatpak/flatpak/issues/1428

Versions of glibc starting with 2.27 include support for memfd_create()
in sys/mman.h, and the included libglnx then drops its internal define.