system_set_harddisk_standby() needs rewritten for current CAM-based ATA stack
The current system_set_harddisk_standby() function in source:etc/inc/system.inc#L2049 uses invalid sysctl values and commands for FreeBSD 10.x.
The old style of setting ATA standby is gone, there is no general way to set standby that I could see. The value must be set using camcontrol, for example:
camcontrol standby ada0 -t x
Where x is the value for standby in seconds, or 0 to disable. It must be run for each ATA device explicitly.
So rather than setting one value we'll likely need to iterate kern.disks and run the camcontrol command for each.