launch: Add support for communicating with Flatpak services
This lets us work around flatpak-spawn not closing file descriptors (https://github.com/flatpak/flatpak-xdg-utils/pull/37) by bypassing it, if we want to.
/cc @denittis
I haven't actually tried this from inside a Flatpak environment, but the equivalent of flatpak-spawn --host
works fine from outside a container.