Eliminate call to swapinfo on FreeBSD
Eliminate call to swapinfo on FreeBSD

Improve performance by eliminating the fork out to swapinfo on FreeBSD which also helps prevent crashes / hangs due to the outstanding fork crash bug:
golang/go#15658

This also fixes the value reported by SwapMemory and SwapMemoryWithContext on FreeBSD which previously only included the first swap device and also reported the values in terms of 1K blocks instead of bytes.

influxdb/gopsutil,influxdb/gopsutil