Ongoing crusade to separate callback tracing into single-process

Ongoing crusade to separate callback tracing into single-process
single-thread part and thread-safe per-request or per-connection
(forthcoming) routines.  Implement trace_startup() routine and call it
from startup hook handlers instead of the old trace_add routine.  This
completely separates the global variable manipulation from the
request-specific stuff.  Otherwise, it's mostly existing code.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@491153 13f79535-47bb-0310-9956-ffa450edef68