Backport r731358/731388/731594

mod_ext_filter: introduce sane error handling when the filter program
fails to start, including an onfail configuration option.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@732586 13f79535-47bb-0310-9956-ffa450edef68