Add OpenBSD support

I managed to make a clean compile and a few test runs using the
attached patches.

These work on OpenBSD 5.4 at least.  I used the 2.1.4 release of
fio as a basis.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/os/os.h b/os/os.h
index 4b59034..03d1e9a 100644
--- a/os/os.h
+++ b/os/os.h
@@ -17,6 +17,7 @@
 	os_hpux,
 	os_mac,
 	os_netbsd,
+	os_openbsd,
 	os_solaris,
 	os_windows,
 	os_android,
@@ -30,6 +31,8 @@
 #include "os-linux.h"
 #elif defined(__FreeBSD__)
 #include "os-freebsd.h"
+#elif defined(__OpenBSD__)
+#include "os-openbsd.h"
 #elif defined(__NetBSD__)
 #include "os-netbsd.h"
 #elif defined(__sun__)