Backport change to htdbm to warning when trying to use

plaintext passwords on a platform where the server doesn't
support them.

Reviewed by: rpluem, colm


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@393883 13f79535-47bb-0310-9956-ffa450edef68