pressure-vessel: Sync flatpak_envp_cmp() from Flatpak 1.11.1
This function was originally written for pressure-vessel and subsequently added to Flatpak. If we use Flatpak's version, we can stay closer to being in sync with Flatpak's flatpak-utils.c.
A lot has changed since we last did a resync of shared files with Flatpak, so I'm probably going to deal with the parts we actually compile gradually, and then take a "one big commit" approach for the parts that are #if 0
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Edited by Simon McVittie