Add support for OpenBSD 4.6

Add support for OpenBSD 4.6

Add config param header and sysname number for OpenBSD 4.6.

As well, add an additional parameter to a call to ifa_ifwithnet()
when looking up the MTU for an interface to indicate that the call
should use the default routing table. With the advent of OpenBSD
4.6, the system has started to make provisions for multiple routing
tables which included a change to the calling sequence for this
routine.

Change-Id: Idb53efef0e229ca26bd587f2f8cd9df8a0248227
Reviewed-on: http://gerrit.openafs.org/1265
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>