Enhancement: stricter early mountpoint validation in `mount`

`restic mount` accepted parameters that would lead to a FUSE mount operation
failing after having done computationally intensive work to prepare the mount.
The `mountpoint` argument supplied must now refer to the name of a directory
that the current user can access and write to, otherwise `restic mount` will
exit with an error before interacting with the repository.

https://github.com/restic/restic/pull/5718