More changes to bring the performance tuning guide closer to compatibility

with httpd-2.0:
 - Updated the sample system call trace with a 2.0/worker example
 - Removed the section on the preforking model


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@95719 13f79535-47bb-0310-9956-ffa450edef68