add --disable-scanner to backup command

The scanner process has only cosmetic effect for the progress printer,
and can be disabled without impacting functionality when the user does
not need an estimate of completion.

In many cases the scanner process can provide beneficial priming of
the file system cache, so as general advice it should not be disabled.
However, tests have shown that backup of NFS and fuse based filesystems,
where stat(2) is relatively expensive, can be significantly faster
without the scanner.

changelog/unreleased/pull-3931

@@ -0,0 +1,8 @@
+Enhancement: Allow backup file tree scanner to be disabled
+
+Restic walks the file tree in a separate scanner process to find the total size
+and file/directory count, and uses that to provide an ETA.  This can slow down
+backups, especially of network filesystems.  The new flag `--no-scan`
+can be used to speed up such backups.
+
+https://github.com/restic/restic/pull/3931