build-export: Canonicalize file permissions
build-export: Canonicalize file permissions

This canonicalizes the file modes for directories to 0755, and
for files to 0755 for executables or 0644 otherwise.

This means we never get files/dirs writable by non-root in the
system repo, and we never get setuid/setgid/sticky-bit, all
which could potentially be a problem if we store them in the
system repo.

See https://github.com/flatpak/flatpak/pull/837 for some discussion
about this.

flatpak/flatpak,GeorgesStavracas/flatpak,flatpak/flatpak,handsome-feng/flatpak,matthiasclasen/flatpak,flatpak/flatpak,handsome-feng/flatpak,handsome-feng/flatpak,matthiasclasen/flatpak,matthiasclasen/flatpak,GeorgesStavracas/flatpak,GeorgesStavracas/flatpak,matthiasclasen/flatpak,flatpak/flatpak,GeorgesStavracas/flatpak,GeorgesStavracas/flatpak,handsome-feng/flatpak,handsome-feng/flatpak,flatpak/flatpak,matthiasclasen/flatpak