Prefer hook_form_BASE_FORM_ID_alter for hook_form_node_form_alter instead of traversing the $form with strpos

hook_form_BASE_FORM_ID_alter: Provide a form-specific alteration for shared forms.
hook_form_BASE_FORM_ID_alter(&$form, &$form_state, $form_id)
http://api.drupal.org/api/function/hook_form_BASE_FORM_ID_alter/7

strpos is inefficient and dispreferred to hook_form_BASE_FORM_ID_alter.

This improvement was suggested by timplunkett in #drupal-contrib