Support falling back to FUSE 2.x

[Overv]

FUSE 3.x is used primarily because of its support for performance enhancing features like kernel write-back caching, but version 2.x (libfuse2) is far more likely to be installed by default. Because it's the only non-Python dependency I think it would be nice if outrun can support falling back to 2.x to simplify the installation.