Add a ignore_unknown node-level directive.

Hi all,

This patch adds a node-level directive to ignore unknown values when running munin-limits.

Use case: I have some clients which go down often (think HTPC), which now causes all plugins with configured limits (such as if_ethX_err) to generate a warning as soon as I power down the device. This patch adds an option to ignore such unknown values.

Although using munin on such clients might not seem that useful, I use it to monitor my disks (both usage and SMART values) which is still quite relevant if the device is off-line frequently.