Extend Header add|set|append support to accept format strings for the

header values.  Configure thusly:

Header add MyHeader "%t %D yadda"

%t is the time the request was received relative to CUT since the epoch measured in microseconds.
%D is the time delta from the time the request was received to the time the headers are sent
on the wire. To facilitate parsing the header response, %D results in the string D=xxx and
%t results in the string t=xxx. Thus the config directive above will result in this header
being included in the response:

MyHeader: t=991424704447256 D=3775428 yadda


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@89255 13f79535-47bb-0310-9956-ffa450edef68