backward compatibility for old authtype settings
add capability to restrict recipients of dokuwiki 'msg' alerts.  This is useful where message is added to the queue before authentication is initialized
restrict 'authtype deprecated' alert to superusers only
for completeness (& defensive coding), test ['show'] against MSG_PUBLIC in case its redefined to a non-zero value
change nomenclature from 'show' to 'allow' (fn from canshow to allowed)
Merge pull request #210 from splitbrain/authcomp_msg

mechanism supporting "authtype deprecated" message for superusers only
Merge pull request #199 from splitbrain/authcomp

backward compatibility for old authtype settings