Add sysctl plugin

Hi,

We have developed at Fotolia a sysctl plugin, allowing us to fetch some values only availaible from sysctl and we would like to merge it upstream.

The plugin is running smoothly on Linux and FreeBSD, but the code may require some modifications to make it portable through multiple OS (like net|openBSD) by checking the existence of function *sysctlbyname(3)*, than I don't know how to do.
Not sure I understand the point of this presence/absence of `<Instance>` blocks. It seems to introduce a fair amount of code just to ensure things work in both cases. Wouldn't it make thinks simpler to just rip out the "legacy" case ?