merge this from 2.1-dev:

    Lower the severity of the "listener thread didn't exit" message
    to debug, as it is of interest only to developers.

PR:              9011
Reviewed by:	 stoddard, brianp


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/APACHE_2_0_BRANCH@101169 13f79535-47bb-0310-9956-ffa450edef68