master: validate multigraph argument

Specially crafted input can lead to munin-html going into
an infinite loop, and OOM the host

Thx Christoph Biedl (debian.axhn@manchmal.in-ulm.de) for noticing
and for providing a patch
master: avoid an endless loop in HTML generation

Safeguard to avoid any bigger loop than 100 nested levels. It protects against
application bugs in the parsing code.

Thx Christoph Biedl (debian.axhn@manchmal.in-ulm.de) for noticing
and for providing a patch
master: don't abort node collection on bad plugin

As noted by Christoph Biedl in the ticket :

Using "multigraph" as a multigraph service name is not accepted by
Munin::Master::Node. The check however dies (in config) or croaks (in fetch),
causing in the connection being dropped and no data gathered for that node.

That's a bit harsh, this commit lowers that situation to two error messages.

Note that impact is rather small as this affects the data for that node itself.
A malicious plugin still does harm, but if an evil guy is able to control which
plugins are run on a node, no longer collecting munin data for that one is not
one of the bigger concerns.

[ Thx Christoph to even provide the patch ]

Closes: #1397
Merge remote-tracking branch 'bundle/fix-cve-oom-html' into debian-wheezy
releasing version 2.0.6-4+deb7u2