Enhancement: Allow `backup` file tree scanner to be disabled

The `backup` command walks the file tree in a separate scanner process to find
the total size and file/directory count, and uses this to provide an ETA. This
can slow down backups, especially of network filesystems.

The command now has a new option `--no-scan` which can be used to disable this
scanning in order to speed up backups when needed.

https://github.com/restic/restic/pull/3931
