Initialize allow_lockdiscovery as unset

Initialize allow_lockdiscovery as unset

Removing the initialization means it is set as 0 by apr_pcalloc(), and
0 is DAV_ENABLED_UNSET

That is required to inherit configuration in dav_merge_dir_config()
as pointed out by rpluem@

We only check for allow_lockdiscovery against DAV_ENABLED_OFF, hence
DAV_ENABLED_UNSET is equivalent to DAV_ENABLED_ON, which is required
for backware compatibility sake.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1905206 13f79535-47bb-0310-9956-ffa450edef68