When the -p parameter is given without an argument, the password is read
from the command line. This allows passwords to be given to MaxCtrl in a
safer manner.
When MaxAdmin would be used without a controlling terminal, it would
refuse to accept passwords from stdin as it could not set the terminal
attributes. This means that executing MaxAdmin commands from other
programs would fail if the process had no controlling terminal.
Turning the error into a warning will allow users to know that terminal
echo is still enabled before they type their passwords.
