Extend mod_headers to support conditional driven Header

Extend mod_headers to support conditional driven Header
add, append and set. Use SetEnvIf to set an envar and conditionally
add/append/set headers based on this envar thusly:

     SetEnvIf TSMyHeader value HAVE_TSMyHeader
     Header add MyHeader "%t %D" env=HAVE_TSMyHeader

If the request contains header "TSMyHeader: value" then header
MyHeader: "t=xxxxxxxxxx D=yyyy" will be sent on the response.

Update mod_headers.html.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@89271 13f79535-47bb-0310-9956-ffa450edef68