wgengine/magicsock,ipn/ipnlocal: store and load homeDERP from cache (#19491)

With netmap caching, the home DERP of the self node was neither saved to
the cache or loaded from it, making nodes not stick to a DERP when
starting without a connection to control.

Instead, make sure that when a cache is available, load that cache,
before looking for DERP servers. This is implemented by allowing a skip
of ReSTUN in setting the DERP map (we must have a DERP map before
setting the home DERP), so the DERP from cache will set itself and be
sticky until a connection to control is established.

Making DERP only change when connected to control is handled by existing
code from f072d017bd.

Updates #19490

Signed-off-by: Claus Lensbøl <claus@tailscale.com>

wgengine/bench/wg.go

@@ -156,8 +156,8 @@ func setupWGTest(b *testing.B, logf logger.Logf, traf *TrafficGen, a1, a2 netip.
 	})
 
 	// Not using DERP in this test (for now?).
-	s1.MagicSock.Get().SetDERPMap(&tailcfg.DERPMap{})
-	s2.MagicSock.Get().SetDERPMap(&tailcfg.DERPMap{})
+	s1.MagicSock.Get().SetDERPMap(&tailcfg.DERPMap{}, true)
+	s2.MagicSock.Get().SetDERPMap(&tailcfg.DERPMap{}, true)
 
 	wait.Wait()
 }