allows passing open /dev/fuse file descriptors so that the FUSE process
can run fully unprivileged. uses the /dev/fd/N mountpoint format from
libfuse3.
macfuse 4.x turns out to be incompatible with the old mounting method where
you open the device by yourself and only supports the newer method where you
receive a file descriptor from `mount_macfuse` through a unix socket.
Co-authored-by: Vitaliy Filippov <vitalif@yourcmc.ru>
Avi