all: add Node.HomeDERP int, phase out "127.3.3.40:$region" hack [capver 111]

This deprecates the old "DERP string" packing a DERP region ID into an
IP:port of 127.3.3.40:$REGION_ID and just uses an integer, like
PeerChange.DERPRegion does.

We still support servers sending the old form; they're converted to
the new form internally right when they're read off the network.

Updates #14636

Change-Id: I9427ec071f02a2c6d75ccb0fcbf0ecff9f19f26f
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

wgengine/magicsock/magicsock_test.go

@@ -314,7 +314,7 @@ func meshStacks(logf logger.Logf, mutateNetmap func(idx int, nm *netmap.NetworkM
 				Addresses:  addrs,
 				AllowedIPs: addrs,
 				Endpoints:  epFromTyped(eps[i]),
-				DERP:       "127.3.3.40:1",
+				HomeDERP:   1,
 			}
 			nm.Peers = append(nm.Peers, peer.View())
 		}