Ensure that min/max valid timestamps (milliseconds since the epoch)

make sense:  no negative numbers, and require an input of "-" instead
of "0" to indicate that the timestamp isn't being provided.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1595034 13f79535-47bb-0310-9956-ffa450edef68