mod_proxy_hcheck: Fix issues with interval determination. PR 62318

trunk patch: http://svn.apache.org/r1838937
2.4.x patch: svn merge -c 1838937 ^/httpd/httpd/trunk .
+1: jim, ylavic, minfrin


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1841266 13f79535-47bb-0310-9956-ffa450edef68