Fuse: Switch to using the "fuse_main()" API
The way we used to use libfuse was somehow first "deprecated" and then
entirely removed from their repository on GitHub. Unfortunately, the old
API worked really elegantly for us (since libfuse calls us from threads
that it created on its own), and with the "fuse_main()" API we require
quite a bit of very verbose extra legwork, but what can you do.
The old way of using libfuse also triggered a bug (SEGFAULT) in libfuse
starting with at least version 2.9.2, while the new "fuse_main()" use
Fuse: Return -ENOSYS in case a Fuse operation errors on Pike level
To achieve this, call_with_interpreter() now catches errors and passes
them on for handling.