Update Flatpak-derived code from Flatpak 1.14.1

Based on !523 (merged).

The majority of the changes are in code that is #if 0 because it isn't needed in pressure-vessel.

Changes that are not #if 0 or trivial:

• don't bind-mount /efi into containers, same as /boot

• slightly better debug if a mkdir() fails in flatpak_context_export()

• instead of deleting code that Flatpak needs but we don't, #if 0 it, to make future merges simpler

• use a better XML DOCTYPE for fonts.conf