OpenBSD: Use Darwin version of afsi_SetServerIPRank() for OpenBSD 4.7 and above.

OpenBSD: Use Darwin version of afsi_SetServerIPRank() for OpenBSD 4.7 and above.

With OpenBSD 4.7, the ia_subnet and ia_subnetmask fields have been removed
from the in_ifaddr structure as part of a cleanup that considers them
unnecessary. Thus, the version of afsi_SetServerIPRank() that OpenBSD was
using no longer compiles. On the other hand, the Darwin version of this
routine seems suited to OpenBSD's needs since it doesn't use those fields
so we'll use that version for OpenBSD 4.7 and above instead.

Change-Id: I27333056378a023e175bddeb637288fc6d1b6e88
Reviewed-on: http://gerrit.openafs.org/3405
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>