refs #803

closes #1139

autopep8 -i --select=W602
Also some other minor deprecated syntax changes, like "while 1".
I did not commit the less obvious changes.

This is recommended by Debian.
closes #618

Florent authored Apr 27, 2012 and Romain Bignon committed Apr 27, 2012

Example of use case (in a cron) :
boobmsg export_all -n -1 -f atom -b presseurop > /var/www/press.atom

And a cool full atom feeds can be broadcasted to all
rss readers.

Laurent Bachelier authored Mar 22, 2012 and Romain Bignon committed Mar 23, 2012

Most backends don't set it nb_messages and no application used it.
No backends set nb_unread and only one application tried to use it.

This should help avoid useless fillobj() calls in some cases.

fixes #827

While this changes constants names, I think it is important as "accused" is
complely wrong. E-mails use the term "Read-receipts" so I chose
to go with RECEIVED.