Changes necessary to document MPMs.

In particular, we use an mpm_common file for directives shared between
mpms.  These changes allow for an

<directivesynopsis location="mpm_common">
<name>Listen</name>
</directivesynopsis>

to be included in mpm documentation files.  Also, I added the option
of a <modulelist>, which is used in documenting directives inside
mpm_common.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@93578 13f79535-47bb-0310-9956-ffa450edef68