many: add integrated snapfuse

[zyga]

This branch adds an integrated copy of squashfuse as the snapfuse executable.
This allows us to not depend on additional executables and thus introduce new
functionality without new (debian) dependencies which are breaking some
invocations of apt update logic.

[niemeyer]

There are now two different PRs doing this. I'd like to give a harder try at importing the code into the vendor directory without us having to carry it all the time.