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.

Reviewed-on: http://gerrit.openafs.org/3405
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 2d2ac3ca5fbd221d297861c0b5c409e9e004545c)
Change-Id: I60c8aeb69484c87b10cdda8a317702d2f80ee849
Reviewed-on: http://gerrit.openafs.org/3453