cxgbe(4):  Keep track of the clusters that have to be freed by the
custom free routine (rxb_free) in the driver.  Fail MOD_UNLOAD with
EBUSY if any such cluster has been handed up to the kernel but hasn't
been freed yet.  This prevents a panic later when the cluster finally
needs to be freed but rxb_free is gone from the kernel.

MFC after:	1 week
Fake out the deprecation of XDEV and XDEV_ARCH by
continuing to use the variables in the configuration
file, but switch XDEV= and XDEV_ARCH= to TARGET= and
TARGET_ARCH= appropriately.

Sponsored by:	The FreeBSD Foundation