Limit ap_pregsub() to 64K, add ap_pregsub_ex() for longer strings and with

better error reporting. Modify ap_varbuf_regsub() to be similar to
ap_pregsub_ex().


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1188950 13f79535-47bb-0310-9956-ffa450edef68