lnd: properly switch RPC port with target net change

With this commit, support for changing the target network (testnet,
simnet, etc) has been finalized. Previously a command line option was
present to swap networks, but the RPC port wouldn’t automatically be
updated to reflect the network.

params.go

@@ -0,0 +1,33 @@
+package main
+
+import "github.com/roasbeef/btcd/chaincfg"
+
+// activeNetParams is a pointer to the parameters specific to the currently
+// active bitcoin network.
+var activeNetParams = segNetParams
+
+// netParams couples the p2p parameters of a network with the corresponding RPC
+// port of a daemon running on the particular network.
+type netParams struct {
+	*chaincfg.Params
+	rpcPort string
+}
+
+// testNetParams contains parameters specific to the 3rd version of the test network.
+var testNetParams = netParams{
+	Params:  &chaincfg.TestNet3Params,
+	rpcPort: "18334",
+}
+
+// segNetParams contains parameters specific to the segregated witness test
+// network.
+var segNetParams = netParams{
+	Params:  &chaincfg.SegNet4Params,
+	rpcPort: "28902",
+}
+
+// simNetParams contains parameters specific to the simulation test network.
+var simNetParams = netParams{
+	Params:  &chaincfg.SimNetParams,
+	rpcPort: "18556",
+}