Support fully static runtimes

[TheAssassin]

Pre-submit checks

• I checked for similar issues beforehand, but could not find any. I could not add my proposal to any existing issue.
• I am going to take the time to to fill in all the required details. I know that the bug report will be dismissed otherwise.

Feature description

See AppImage/AppImageSpec#38 for all the details.

Proposed solution

Once an AppImage spec type 2.1 is released, we can reliably detect that and know that $TARGET_APPIMAGE is available as a feature. We then do not need to use $LD_PRELOAD any more at all.

Alternative solutions you considered

No response

Additional context

No response

[probonopd]

Once an AppImage spec type 2.1 is released, we can reliably detect that and know that $TARGET_APPIMAGE is available

Would running something like strings ... | head ... | grep "TARGET_APPIMAGE" be a reasonable detection, so that it would work without the need for a type 2.1?

[TheAssassin]

We can do it properly. Why resort on such a hack?

Not to mention that this additional version marker is a great test run for the already-WIP type 3, in which the magic bytes will be moved in a similar way to another location, too.

I'm open to alternative suggestions, but please let us use the spec issue for coordination.