prefork MPM: Ignore mutex errors during graceful restart. For

certain types of mutexes (particularly SysV semaphores), we
should expect to occasionally fail to obtain or release the
mutex during restart processing.

These mutex failures seem to be a pretty rare occurrence, but
it was fatal.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@95665 13f79535-47bb-0310-9956-ffa450edef68