Minor coding style fixes

This makes a number of changes:
  * Remove the dependency on marshal by using string.dump and loadstring.
  * Use lua_tolstring rather than having Lua functions pass string lengths to C++.
  * Move lua_api/l_async_events.* to cpp_api/s_async.*, where it belongs.
  * Make AsyncWorkerThread a child of ScriptApiBase, this removes some duplicate functionality.
  * Don't wait for async threads to shut down.  (Is this safe?  Might result in corruption if the thread is writing to a file.)
  * Pop more unused items from the stack
  * Code style fixes
  * Other misc changes

Add support for (configurable) multiline hotbar
Improved screensize handling
Add userdefined gui scale by BlockMen

This reverts commit 5b518ed2.

This caused duplicate tracebacks and tracebacks when unwanted.
It also ignored the level argument to error() and didn't pass the message (or level) to debug.traceback().
Use xpcall() or lua_pcall()'s errorhandler argument instead.

This reverts commits a2003b0d and 54ffe2e5.

These weren't correct.  Add a black magic warning instead.

Thats updated #795

Add log entry for peer timeout

add c++11 atomic support (optional)