Add FreeBSD support

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

rwcancel/fdset_freebsd.go

@@ -0,0 +1,22 @@
+/* SPDX-License-Identifier: GPL-2.0
+ *
+ * Copyright (C) 2017-2018 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
+ */
+
+package rwcancel
+
+import "golang.org/x/sys/unix"
+
+type fdSet struct {
+	fdset unix.FdSet
+}
+
+func (fdset *fdSet) set(i int) {
+	bits := 32 << (^uint(0) >> 63)
+	fdset.fdset.X__fds_bits[i/bits] |= 1 << uint(i%bits)
+}
+
+func (fdset *fdSet) check(i int) bool {
+	bits := 32 << (^uint(0) >> 63)
+	return (fdset.fdset.X__fds_bits[i/bits] & (1 << uint(i%bits))) != 0
+}