add GlobalLog directive to allow a diagnostic log to be inherited

by all virtual hosts, even if they define their own logs.

Submitted By: Edward Lu <Chaosed0 gmail.com>
Committed by: covener



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1599535 13f79535-47bb-0310-9956-ffa450edef68