wgengine/userspace: add support to automatically enable/disable the tailscale

protocol in BIRD, when the node is a primary subnet router as determined
by control.

Signed-off-by: Maisem Ali <maisem@tailscale.com>

docs/bird/sample_bird.conf

@@ -0,0 +1,16 @@
+log syslog all;
+
+protocol device {
+  scan time 10;
+}
+
+protocol bgp {
+  local as 64001;
+  neighbor 10.40.2.101 as 64002;
+  ipv4 {
+    import none;
+    export all;
+  };
+}
+
+include "tailscale_bird.conf";