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.

doc/040_backup.rst

@@ -204,8 +204,17 @@ Combined with ``--verbose``, you can see a list of changes:
     modified  /archive.tar.gz, saved in 0.140s (25.542 MiB added)
     Would be added to the repository: 25.551 MiB
 
-.. _backup-excluding-files:
+Disabling Backup Progress Estimation
+************************************
 
+When you start a backup, restic will concurrently count the number of
+files and their total size, which is used to estimate how long it will
+take.  This will cause some extra I/O, which can slow down backup of
+network file systems or fuse mounts.
+
+-  ``--no-scan``  Do not run scanner to estimate size of backup
+
+.. _backup-excluding-files:
 Excluding Files
 ***************