mod_rewrite: Fix some problems reporting errors with mapping

programs (RewriteMap prg:/something).

the wrong field was specified when trying to log the name of
the program that couldn't be started

recent APR features used to provide better error reporting
on systems where apr_proc_create() uses fork()


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@98840 13f79535-47bb-0310-9956-ffa450edef68