Fix a problem with security relevant file rights

The file /etc/nullmailer/remotes is a file where also password could be
configured. Therefore the original file is owned by mail and only
readable by user mail.

If there is a global setting for owner and mode as defaults for file as
it is very common, this would leak the data to the world. Setting that
explicit for the file fixes this issue.
Also make the 'adminaddr' file protected.

The address that is being sent to might also be sensitive, so make
the permissions and ownership tighter.
Add Klaus Ethgen to contributors

Thanks for identifying a permission problem with this module
Clarify the English some more.
Update copyright years
Release v1.3