support building against recent libfuse v2.x or older v3.x

[chrisd8088]

Requiring use of libfuse version 3.x may be barrier to adoption for some for whom their Linux (or BSD?) distribution only has version 2.x installed by default. This may be particularly true for those running Long-Term Support distro versions.

What would be great (not urgent, but a nice-to-have) would be to adapt our code to the best available FUSE library, either auto-detected by autoconf or overridden with a --with-fuse=/path/to/lib sort of configure flag.

[kivikakk]

I had a look into this, and it appears to be quite a tall order. After changing includes and linkage around, here's a sneak peak of what it looks like to try to compile:

It goes on and on. Rather than attempt any further, I've saved the diff of my WIP at https://gist.github.com/kivikakk/fabd775ceecbf4380757833080a6b278 and will leave it there for now.