Convert mod_auth_digest to use the new apr_global_mutex_t type.

Added some comments that will help us make this more portable.
tmpnam() is not portable to platforms that will not inhereit
the process, so we need to pull the lock filename from somewhere
shared, like a configuration directive.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@93533 13f79535-47bb-0310-9956-ffa450edef68