cleanups related to new AllowOverrideList functionality:

- add new NOT_IN_HTACCESS flag for ap_check_cmd_context() 
- describe the need for this in new_api_2_4.xml
- forbid Define and UnDefine in .htaccess


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1153676 13f79535-47bb-0310-9956-ffa450edef68