CRM-19303 - Drupal - Fix various path computations for multisite

CRM-19303 - Drupal - Fix various path computations for multisite

A few changes:
 * Move logic out of `CRM_Utils_System_Base` to `CRM_Utils_System_DrupalBase`
   so that it's easier to read.
 * Update `[civicrm.files]`:  First, try to use `sites/{$name}/files/civicrm`.
   Otherwise, use `sites/default/files/civicrm`.
    * Note: The `$name` is parsed/validated the same way as `civicrm.config.php`.
      It takes the full HTTP_HOST and then tries various substrings.
 * Update `[civicrm.root]`:  Compare `$civicrm_root` and `cmsRootPath()`
   to determine the relative path.  Then, combine `CIVICRM_UF_BASEURL` with
   relpath.  (This is the same as WordPress.)