Add FreeBSD support

Signed-off-by: Brady OBrien <brady.obrien128@gmail.com>

rwcancel/select_default.go

@@ -0,0 +1,14 @@
+/* SPDX-License-Identifier: GPL-2.0
+ *
+ * Copyright (C) 2017-2018 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
+ */
+
+// +build !linux
+
+package rwcancel
+
+import "golang.org/x/sys/unix"
+
+func unixSelect(nfd int, r *unix.FdSet, w *unix.FdSet, e *unix.FdSet, timeout *unix.Timeval) error {
+	return unix.Select(nfd, r, w, e, timeout)
+}