add initial envsys support for NetBSD
The current NetBSD support doesn't compile, due to lack of APM on my AMD64 machine. This MR adds support for the envsys(9) framework, which allows monitoring hardware in a platform independent way (i.e., it abstracts away the ACPI sensors of my laptop).
I tried to change as little as possible and re-use as much of the existing code as possible. I tested this patch on a NetBSD 10.0 RC1 laptop. However,I haven't tested it on a Linux box, yet. For NetBSD, there is now a dependency on libprop (which is part of the base NetBSD install).
I haven't touched autoconf in 15 years, so my changes to the *.am files may be not the most elegant.
TODO: sigh, of course the autoconf stuff doesn't work in the Linux build. Will fix that. Marked as draft until I get around to it