Backout r410758 because DavLockDB needs to point to an httpd-writable

directory and @exp_runtimedir@ doesn't qualify.  Also document a
missing module requirement.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@424817 13f79535-47bb-0310-9956-ffa450edef68