Add a "kern.features.ata_cam" sysctl in the kernel when the ATA_CAM kernel
option is defined. This sysctl can be queried by feature_present(3).

Query for this feature in /sbin/atacontrol and /usr/sbin/burncd.
If these utilities detect that ATA_CAM is enabled, then these utilities
will error out. These utilities are compatible with the old ATA
driver, but are incomptible with the new ATA_CAM driver. By erroring out,
we give end-users an idea as to what remedies to use, and reduce the need for them
to file PR's. For atacontrol, camcontrol must be used instead,
and for burncd, alternative utilties from the ports collection must be used
such as sysutils/cdrtools.

In future, maybe someone can re-write burncd to work with ATA_CAM,
but at least for now, we give a somewhat useful error message to end users.

PR: 160979
Reviewed by: jh, Arnaud Lacombe <lacombar at gmail dot com>
Reported by: Joe Barbish <fbsd8 at a1poweruser dot com>
MFC after: 3 days