Follow up to 1892038, 1892063.

Follow up to 1892038, 1892063.

Improve fix to please a fuzzer which reports:
   util.c:2713:26: runtime error: signed integer overflow:
   9999999999999999 * 1000 cannot be represented in type 'long'

Compute the maximum limit for each case 's', 'h', 'ms' and 'mi' and make sure that the input is below this value.

While at it, move a comment to make things more consistent and use 'apr_time_from_msec() instead of hand writing it.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1892185 13f79535-47bb-0310-9956-ffa450edef68