Add default value for DBDExpire, DBDKeep, DBDMin and DBDMax.

Pointed out by: Andrew Ford <A.Ford ford-mason.co uk>
(backport of r657498)

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@657525 13f79535-47bb-0310-9956-ffa450edef68