Log CGI/FCGI Last-Modified header value changes.

The Last-Modified header coming from a backend FCGI/CGI script is inspected
by util_script.c to enforce RFC2616 (https://tools.ietf.org/html/rfc2616#section-14.29).
The Last-Modified header also needs to be compliant with RFC882/1123 as stated in
https://tools.ietf.org/html/rfc2616#section-3.3.1, and one important assumption that
httpd makes (correctly, as the RFC suggests) is to assume the GMT timezone. If the datestr
returned by the FCGI/CGI script is set with a different timezone, then the value might be considered
"in the future" and replaced with GMT now() as calculated by httpd. Adding a trace log might
help sysadmins while debugging these kind of issues. This is a follow up of r1748379. 
  


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1750747 13f79535-47bb-0310-9956-ffa450edef68