Since version 3 of fuse libfuse no longer does anything when given the
nonempty option and it's default is to allow mounting over non empty
directories like normal mount does.
Some versions of libfuse give an error when using `--allow-non-empty`
which is annoying for the user.
We now do this check ourselves so we no longer need to pass the option
to libfuse.
Fixes#3562
Before this change, the device name was always the remote:path rclone
was configured with. However this can contain sensitive information
and it appears in the `mount` output, so `--devname` allows the user
to configure it.
See: https://forum.rclone.org/t/rclone-mount-blomp-problem/29151/11
