Implement mod communication channels (#6351)

Implement network communication for channels

* Implement ModChannel manager server side to route incoming messages from clients to other clients
* Add signal handler switch on client & ModChannelMgr on client to handle channels
* Add Lua API bindings + client packet sending + unittests
* Implement server message sending
* Add callback from received message handler to Lua API using registration method

src/script/lua_api/l_modchannels.h

@@ -0,0 +1,66 @@
+/*
+Minetest
+Copyright (C) 2017 nerzhul, Loic Blot <loic.blot@unix-experience.fr>
+
+This program is free software; you can redistribute it and/or modify
+it under the terms of the GNU Lesser General Public License as published by
+the Free Software Foundation; either version 2.1 of the License, or
+(at your option) any later version.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU Lesser General Public License for more details.
+
+You should have received a copy of the GNU Lesser General Public License along
+with this program; if not, write to the Free Software Foundation, Inc.,
+51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+*/
+
+#pragma once
+
+#include "lua_api/l_base.h"
+#include "config.h"
+
+class ModChannel;
+
+class ModApiChannels : public ModApiBase
+{
+private:
+	// mod_channel_join(name)
+	static int l_mod_channel_join(lua_State *L);
+
+public:
+	static void Initialize(lua_State *L, int top);
+};
+
+class ModChannelRef : public ModApiBase
+{
+public:
+	ModChannelRef(ModChannel *modchannel);
+	~ModChannelRef() = default;
+
+	static void Register(lua_State *L);
+	static void create(lua_State *L, ModChannel *channel);
+
+	// leave()
+	static int l_leave(lua_State *L);
+
+	// send(message)
+	static int l_send_all(lua_State *L);
+
+	// is_writeable()
+	static int l_is_writeable(lua_State *L);
+
+private:
+	// garbage collector
+	static int gc_object(lua_State *L);
+
+	static ModChannelRef *checkobject(lua_State *L, int narg);
+	static ModChannel *getobject(ModChannelRef *ref);
+
+	ModChannel *m_modchannel = nullptr;
+
+	static const char className[];
+	static const luaL_Reg methods[];
+};