Compatiblity with type2 runtime

This issue is just to keep track of the differences wrt to the current type2 runtime and see what will eventually be done for each case.

--appimage-extract is already implemented, however the resulting directory gets named AppDir instead of squashfs-root so this breaks compatiblity with scripts/programs tha expect a squashfs-root when extracting an AppImage. So there are two choices here:

Either keep the name or guarantee that compatibility.

However calling it squashfs-root is also wrong since it can be a dwarfs image instead. 😅

Also --appimage-extract <regex> isn't supported either, so doing --appimage-extract *desktop will extract the entire AppImage instead of just the top level .desktop file.

Here are some other missing features:

Checking for the env variable APPIMAGE_EXTRACT_AND_RUN=1 or the flag --appimage-extract-and-run which makes the appimage self extract on ${TMPDIR:-/tmp} and run, this is often used in CI to avoid issues with fuse not working.
export the env variables ARGV0 and APPIMAGE. There is also APPDIR and OWD but those are rarely used.
- check for a $0.home or $0.config directory which is used for the portable $HOME or $XDG_CONFIG_HOME in the type 2 runtime
- --appimage-updateinfo which just displays the embedded update information, as far as I know appimageupdatetool doesn't use the flag to update and directly checks the appimage instead.
--appimage-signature which I'm not sure if validate needs it in order to work
--appimage-offset and --appimage-mount which I have never used lol

Edit: The type2-runtime also checks the FUSERMOUNT_PROG env variable which allows it to work when fuse is on a different location outside PATH.

VHSgunzo / uruntime

Compatiblity with type2 runtime #1