Restart changes listener on config changes

Previously if chttpd_auth/authentication_db was changed in the
config then a changes listener would not be started for the new
authentication DB until the current changes request timed out.
During that time any changes to the users DB (e.g. password
changes) would not take effect. This is primarily a problem when
running share/www/script/test/users_db_security.js however it could
conceivably become a problem under normal running conditions.

This commit adds a config listener which causes the current changes
listener to be killed when the chttpd_auth/authentication_db config
value is changed. It will then be restarted via the existing
handle_info/2 clause.