Some "use chroot" improvements.

Some "use chroot" improvements.

- The sanitize_paths variable was set too often. It only needs to be set
  when the "inner" path is not "/".  This change avoids sanitizing &
  munging things for a path=/ module just because chroot is off.
- The default for "use chroot" is now "unset" instead of "true".  When
  unset it checks if chrooting works, and if not, it proceeds with a
  sanitized copy instead of totally failing to work.  This makes it
  easier to setup a non-root rsync daemon, for instance.  It will have
  no effect on a typical Linux root-run daemon where the default will
  continue to use chroot (because chrooting works).  A config file can
  explicitly set "use chroot = true | false" to force the choice.
- Try to improve the "use chroot" manpage.