Unbreak building for FreeBSD (#103)

Add support for freebsd. You have to set `tun.dev` in your config. The second pass of this would be to remove the exec calls and use ioctl(2) and route(4) instead, but we can do that in a second PR.

Co-authored-by: Wade Simmons <wade@wades.im>

udp_freebsd.go

@@ -0,0 +1,34 @@
+package nebula
+
+// FreeBSD support is primarily implemented in udp_generic, besides NewListenConfig
+
+import (
+	"fmt"
+	"net"
+	"syscall"
+
+	"golang.org/x/sys/unix"
+)
+
+func NewListenConfig(multi bool) net.ListenConfig {
+	return net.ListenConfig{
+		Control: func(network, address string, c syscall.RawConn) error {
+			if multi {
+				var controlErr error
+				err := c.Control(func(fd uintptr) {
+					if err := syscall.SetsockoptInt(int(fd), syscall.SOL_SOCKET, unix.SO_REUSEPORT, 1); err != nil {
+						controlErr = fmt.Errorf("SO_REUSEPORT failed: %v", err)
+						return
+					}
+				})
+				if err != nil {
+					return err
+				}
+				if controlErr != nil {
+					return controlErr
+				}
+			}
+			return nil
+		},
+	}
+}