Add r->mtime field to reduce gratuitous date parsing, and

	update_mtime() function to advance it as needed.  (This will
	be used by the ETag and Last-Modified changes coming shortly.)

Reviewed by:	Roy Fielding


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@79070 13f79535-47bb-0310-9956-ffa450edef68