[collectd] Enhancements to NetBSD support

[Havard Eidnes]

Hi,

over the last time I've improved the support for collecting
metrics from NetBSD hosts.  I've added plugin support for the
disk, entropy, irq, memory, processes, and swap plugins.

It's time to submit these as pull requests, probably via github.
Do you prefer them all in one go, or separately?  I had to touch
configure.ac to add these, and some of the required changes are
"adjacent" to each other, so it's probably easiest if I submit
them as a singe pull request?

Regards,

- Håvard