* core: Limit ap_pregsub() to 64MB, add ap_pregsub_ex() for longer strings.

    The default limit can be adjusted at compile time using AP_PREGSUB_MAXLEN.
    mod_setenvif: Log error on substitution overflow.


Submitted by: trawick
Reviewed by: trawic, wrowe, covener



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@1497429 13f79535-47bb-0310-9956-ffa450edef68