new borg mount / fuse fs implementation

[ThomasWaldmann]

Currently borg 1.x and master branch both use llfuse and pyfuse3 libraries to implement fuse2 and fuse3 support.

But both libraries are not developed anymore, see #6734.

There is a well-maintained other fuse library:

https://github.com/mxmlnkn/mfusepy/

It works differently (path-based lookups, not inode-based as in the low-level libraries), but I guess we can also work with that, maybe it is even easier that way.

Also, it seems to support fuse(2) and fuse3 in one library.

[ThomasWaldmann]

it works! :-)

maybe found a small bug (only appears if one uses offsets): https://github.com/mxmlnkn/mfusepy/pull/6