fix compile breakage with IBM's xlc compiler caused by the

attempt to lose const on progname

strrchr was not the real library function that takes a
const parameter; instead, strrchr was the Apache macro which
makes it clear that const will be lost


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@95900 13f79535-47bb-0310-9956-ffa450edef68