Make clear password optional

I put a new variable in the configuration of vexim that allows you to switch on and off ($saveclearpw=1/0) the use of clear text passwords. If it is activated, it stores the password like it was before. But I switched it off by default.

The current configuration of exim, courier and dovecot already use the encrypted field. You may be caution with older versions of vexim which might use clear passwords.