Add/generalize abstraction in arc_summary2

Add/generalize abstraction in arc_summary2

A more generic interface can be used for the abstraction of loading
kstats, allowing other platforms to implement the function cleanly.

In a similar vein, loading tunables can be abstracted away in order for
other platforms to provide their own implementations of this function.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Matt Macy <mmacy@FreeBSD.org>
Signed-off-by: Ryan Moeller <ryan@ixsystems.com>
Closes #9277