m/update: fix "not a reference at /../Utils.pm"

As Petter Reinholdtsen noticed :

    The issue is that $data in the call to Storable::nstore() is
    undefined.

    This get rid of the warning in the cron email triggered by trying to store
    nothing in /var/lib/munin/htmlconf.storable.tmp.$pid.  But the cron email
    is still sent out, this time with another and more understandable message:

      [FATAL] There is nothing to do here, since there are no nodes with
      any plugins. Please refer to http://munin-monitoring.org/wiki/FAQ_no_graphs

I commited a slighly edited patch, but the effect is exactly the same as the
one that Petter wrote.

Closes: D:696981
Ensure consistent use of /etc/default/munin-node

Closes: #764594
Add documentation links to systemd services
Merge branch 'bts/696981-fix-not-a-reference-utils-pm' into stable-2.0
Use optional /etc/default/munin-node from systemd service

Thanks: Petter Reinholdtsen
Fix the "apt_all update" cron job

Closes: 761366

This partially reverts commit b80f5f7299ec9444dcd27f39f64286d1471688fa.

The plugin sets the correct plugin state file by itself, so it is not
necessary to revert all it did.
Revert "p/postfix_mail: fix incorrect delivered message count"

This reverts commit d07e7c15d11cebbcdf03c3171f406cd01f39e460.

Reopens: D:675318
p/postfix_mailstats: proper fix, but partial

It only partially handles D:675318, as postfix_mailvolume is not fixed.
That plugin needs a special handling, since the information is on 2 different
lines, and it's not an one-line fix.
Merge branch 'bts/675318-incorrect-postfix-delivery-count' into stable-2.0
ChangeLog: 2.0.24
Declare compliance with Debian policy 3.9.6
update changelog before import
Imported Upstream version 2.0.24
Merge tag 'upstream/2.0.24' into debian

Upstream version 2.0.24
Imported upstream release 2.0.24
releasing package munin version 2.0.24-1
munin.README.Debian: point to the new http://guide.munin-monitoring.org/

Replace link to  http://munin-monitoring.org/wiki/Documentation with one to the new
http://guide.munin-monitoring.org/ which is better structured and also translated.