Introduce a specific pool for stderr logging, so that we have two

simultanious pools until the new stderr pipe logger is completely 
initialized.  This ensures we have a stderr channel from the time 
we clear the plog in a previous generation through the invocation
of the open_logs hook in the next generation.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@569535 13f79535-47bb-0310-9956-ffa450edef68