drush_valid_drupal_root() is not rigorous enough

drush_valid_drupal_root() requires only a file called includes/common.inc to match a Drupal directory, which causes problems when this file pattern appears elsewhere, in non-Drupal root contexts (e.g. in module_builder module).

To fix this problem, this patch enhances the tests done by drush_valid_drupal_root() to also require a file misc/drupal.js.  I picked this file because it occurs in D6, D7 and D8, and it has the name "Drupal" in it.