Provide an udev rule with ID_SERIAL based, disk plugin usable ID attribute

Provide an udev rule with ID_SERIAL based, disk plugin usable ID attribute

Disk plugin UdevNameAttr option is very useful for making sure that the
same disk always ends in the same RRD file no matter which particular
device node it ends being assigned by the OS.

However, using the most fitting ID_SERIAL udev attribute for this purpose
results in data about the whole disk and each partition being mixed
together incorrectly, since this attribute has exactly the same value for
the whole disk device node and its particular partition nodes.

Let's introduce an ID_COLLECTD attribute instead which differentiates
between the whole disk and its particular partitions in a new udev rule
file provided in "contrib" directory.
To warn users about the issue let's also add a relevant note to the
UdevNameAttr option description.

Based on Stewart Adam's workaround from
https://github.com/collectd/collectd/issues/823

Signed-off-by: Maciej S. Szmigiero <mail@maciej.szmigiero.name>