Correct ram_in_use alarm calculation on FreeBSD (#4363)
* There is no "cached" dimension on FreeBSD, use "cache" instead
* Account "buffers" as used memory. As buffers by default don't shrink
much under pressure.
* Account "inactive" as free memory. As pages from inactive list could be
cleared and become free as soon as somebody request more memory from
kernel.
* Sign CLA