Find the karma DB dir (#264)

* Update data_post_headers documentation
We cannot reject at this stage, which is only there to alter headers.
Fix #258

* DMARC plugin: reject in data_post
Followup of #258: we cannot reject a connection during data_post_headers. So add a new hook in data_post to do the real rejection

* Use candidate_dirs to find the DB

plugins/karma_tool

@@ -63,7 +63,7 @@ sub db {
     my ( $self ) = @_;
     return $self->{db} if $self->{db};
     $self->{db} = Qpsmtpd::DB->new( name => 'karma' );
-    $self->{db}->dir(
+    $self->{db}->candidate_dirs(
         $self->{args}{db_dir},
         qw( /var/lib/qpsmtpd/karma ./var/db ./config . ) );
     my $path = $self->{db}->path;