remove references to Irix uslock() mutex primitive

tweak mutex mechanism descriptions to describe them in terms of
AcceptMutex directive instead of ancient 1.3-style CFLAGS for
enabling to the mechanism

PR:         24228


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@101897 13f79535-47bb-0310-9956-ffa450edef68