unix: use sort.Search for sysctl lookup on OpenBSD

Use sort.Search to look up sysctl mib values from names instead of using
a hand-crafted binary search.

Also add a simple test to verify sysctl is still working after the
change.

Change-Id: I844840ac434a704b14c3166c3730daffabd2f347
Reviewed-on: https://go-review.googlesource.com/76930
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

unix/syscall_bsd_test.go

@@ -60,3 +60,11 @@ func TestSysctlRaw(t *testing.T) {
 		t.Fatal(err)
 	}
 }
+
+func TestSysctlUint32(t *testing.T) {
+	maxproc, err := unix.SysctlUint32("kern.maxproc")
+	if err != nil {
+		t.Fatal(err)
+	}
+	t.Logf("kern.maxproc: %v", maxproc)
+}