FBSD: VIMAGE support

FBSD: VIMAGE support

Starting in FreeBSD 8.0, there is support for multiple virtual
network stacks (generally to be exposed to separate jail(8) environments).
It is enabled as a kernel configuration option, so our builds against
GENERIC have not failed, but we fail to build when options VIMAGE
is present.  Fix our variable references accordingly.

Change-Id: I679361b8ea62b0eae90c0aa61287dfc2dd189481
Submitted-by: Hiroki Sato of freebsd.org
Reviewed-on: http://gerrit.openafs.org/4721
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>