Replace kvm_getswapinfo(3) with sysctl call.

This change removes the dependancy on libkvm and allows all sensors
to run from unprivilaged userspace (before access to /dev/kmem was
required to get swap, and other, sensors).

kvm_getswapinfo_sysctl is a heavily modified function from libkvm,
however due to the extensive changes I did not use its original
license.