Based on feedback from dev@httpd, store Vary headers inside the '.header' file.

These Vary headers are used to build a new hash for a URL, based on the 
client's headers.  This allows mod_disk_cache to store many variations of one 
URI, where previously it would only store a single variant.

htcacheclean needs to be updated to understand the new file format.  I plan on 
updating htcacheclean in the next couple days.

PR: 35211


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@190530 13f79535-47bb-0310-9956-ffa450edef68