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): mxmlnkn/mfusepy#6

[ThomasWaldmann]

mfusepy 3.1.0 was just released with a lot of improvements (bug fixes and much improved CI / platform testing).

#9213 was just merged into master and will be in borg 2.0.0b20 for public testing.