gnus: don't tell we are running mbsync in quick mode
files: put auto-save files in a dedicated directory

It seems that the prefix cannot just be a directory. Add something for
that.
gnus: expose user/password through environment variable

In `.mbsyncrc`, we have something like this:

    PassCmd "echo ${PASSWORD}"
gnus: put spaces around mbsync status in modeline
gnus: explain what mbsyncrc should look like