Description
      This is used to implement the --installroot option for installation.  In earlier
      PEAR versions, this was implemented in PEAR_Installer::install(),
      but this makes it more difficult to track with the introduction of channels, and to
      satisfy better encapsulation, it has been moved to PEAR_Config.
     
      Pass in a full path.  On retrieving any directory configuration variable, the
      value will be prepended with the installroot specified in this method.  For example,
      if php_dir is /usr/lib/php, and
      setInstallRoot() is used with /hoopla/boo,
      the value returned from get() would be
      /hoopla/boo/usr/lib/php.  Use an empty string ''
      to reset an installroot.