lncli: add lncli autopilot commands

This commit adds commands 'status', 'enable' and 'disable' for
autopilot.

The commands will only be enabled for autopilotrpc builds of lncli.

cmd/lncli/autopilotrpc_default.go

@@ -0,0 +1,10 @@
+// +build !autopilotrpc
+
+package main
+
+import "github.com/urfave/cli"
+
+// autopilotCommands will return nil for non-autopilotrpc builds.
+func autopilotCommands() []cli.Command {
+	return nil
+}