The mainline port has almost reached full feature parity with the
downstream port. Default to mainline in new installations, but keep
downstream around as it'll still be needed for bringing up the currently
missing features.
This is required for kernels earlier than 4.7 for bwrap to work for
normal users, and therefore for gnome web (epiphany) and flatpak.
[ci:skip-build] [ci:ignore-count]
