More generic check for OpenBSD platform

More generic check for OpenBSD platform

OpenBSD versioning is different from many other
projects, and, say, 5.0 does not differ from 4.9 more than from 5.1. So the
right approach will be to check that platform name starts with "openbsd"
instead. This is also the thing OpenBSD developers do when patching other
software, so this patch is consistent with already existing practicies.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=9888

Reviewed-by: Lars Müller <lars@samba.org>
Reviewed-by: Björn Jacke <bj@sernet.de>
Autobuild-User(master): Björn Jacke <bj@sernet.de>
Autobuild-Date(master): Fri May 17 17:58:16 CEST 2013 on sn-devel-104