mod_privileges: introduce PrivilegesMode: fast mode as before vs secure

mode to fork an unprivileged child per-request in the manner of MPM-ITK
anwhere there's a risk of running malicious code.
Documentation to follow.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@771144 13f79535-47bb-0310-9956-ffa450edef68