common.dtd: add idtype attribute to directive

This change completes r1805372 and also fixes
links generation for <ManageDomain> and <SSLPolicy>
in sections.xml



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1805376 13f79535-47bb-0310-9956-ffa450edef68