OpenBSD: fix lookup of network interfaces

OpenBSD: fix lookup of network interfaces

Starting with OpenBSD 4.2, the interface list returned by sysctl using
NET_RT_IFLIST contains multiple versions of the data. This really
messes up the rx_getAllAddr_internal() routine that returns a list of
valid interfaces for the system (to the point where none are returned).

This change adds a routine that cleans up the data returned by sysctl
so it returns only a single (the most current) version of the data.
That stops afsd (among others) from being cranky when it starts up.

Change-Id: Ief1df0d2c3eb241c45b439a785ddb67d3afe992c
Reviewed-on: http://gerrit.openafs.org/1274
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>