libkvm: Fix kvm_getloadavg() on modern kernel vmcores

[mgorny_gentoo.org]

Summary

Fix kvm_getloadavg() to work correctly on vmcores for modern kernel
versions. The kernels no longer include the _fscale symbol causing
the kvm_nlist() invocation to fail. The code seemed to already assume
that _fscale could be missing but the early kvm_nlist() result check
has caused the function to fail if any symbol were missing. Modify
it to only treat _averunnable as obligatory, and handle missing
_fscale gracefully.

Sponsored by: The FreeBSD Foundation

[mgorny_gentoo.org]

Could you push this for me, as I don't have push access?

[jhb]

Do you happen to have an existing git commit for this pushed somewhere public?

[mgorny_gentoo.org]

I've opened https://github.com/freebsd/freebsd-src/pull/570 for that.