Add OmitPID keyword on Mutex directive to suppress the

Add OmitPID keyword on Mutex directive to suppress the
addition of the parent pid to the generated lock file
name.

Change the Mutex directive syntax from

  Mutex default|mutex-name mechanism

to

  Mutex mechanism [default|mutex-name] ... [OmitPID]

as suggested by wrowe.  (Thanks!)

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@886154 13f79535-47bb-0310-9956-ffa450edef68