FR support exit nodes for tailscaled-on-MacOS

**Describe the bug**
Using https://github.com/tailscale/tailscale/wiki/Tailscaled-on-macOS, exit-nodes do not appear to work.

**To Reproduce**
1. Compile and run tailscaled on MacOS.
1. Use `tailscale up --exit-node=W.X.Y.Z`
1. Go to https://whatismyipaddress.com/ or other means of identifying the IP address

My local IP address is still shown, not the IP address of my exit-node.

**Expected behavior**
To see the IP address of my exit-node.

**Screenshots**
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**Version information:**
- Device: laptop
- OS: MacOS 11.2.3
- OS version: 11.2.3
- Tailscale version: built from HEAD, 2 April 2021

**Additional context**
Add any other context about the problem here.