Simplify the configuration mechanisms for threading in APR.

Revise how Apache configs APR and allow MPMs (or other config scripts) to
    pass cmd line params down to APR. For the MPMs that specifically require
    or disallow threads, have them force APR threads on/off.

Submitted by: Joe Orton <joe@light.plus.com>
Reviewed by: Greg Stein


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@86794 13f79535-47bb-0310-9956-ffa450edef68