Close possible race condition in accept_mutex_init() with O_EXCL

flag.
Reviewed by:	Randy Terbush, Chuck Murcko
Submitted by:	Marc Slemko


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@77367 13f79535-47bb-0310-9956-ffa450edef68