mod_log_config: Make ${cookie}C correctly match whole cookie names

instead of substrings.

PR: 28037
Submitted by: Dan Franklin <dan dan-franklin.com>, Stefan Fritsch


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@833738 13f79535-47bb-0310-9956-ffa450edef68