change the way that thread id is specified in the log format since

the previous implementation used a format string already taken
by mod_logio

now, an optional %P format is used instead

thanks to Andre' Malo for pointing out that I chose a format string
already used by mod_logio!


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@99442 13f79535-47bb-0310-9956-ffa450edef68