Add/generalize abstractions in arc_summary3

Add/generalize abstractions in arc_summary3

Code for interfacing with procfs for kstats and tunables is Linux-
specific. A more generic interface can be used for the abstractions of
loading kstats and various tunable parameters, allowing other platforms
to implement the functions cleanly. In a similar vein, determining the
ZFS/SPL version 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 #9279