library: dont use vm_min_free on non-Linux systems

library: dont use vm_min_free on non-Linux systems

For a specific slice of kernel versions we can get a better
estimate of the available memory before the "real" available
figure appears around kernel 3.2

However, that middle method requires a /proc/sys/vm directory.
FreeBSD 9.x used to emulate a < 2.6.27 kernel procfs meaning
it never tried the middle method. FreeBSD 10.x emulates something
more modern, but without the available figure and without a
/proc/sys/vm, so the library falls into a hole.

Hurd may to one day have this bug so we'll exclude him as well
as its triggered by whatever number appears in
/proc/sys/kernel/osrelease

References:
 commit 3f3b1a59ada85d21f34530cfb1dc1c0afd70e6dc
 https://bugs.debian.org/831396