Introduce builtin_shared and use it to fix #4778

Fixes #4778 which was about the error:

ServerError: Lua: Runtime error from mod '' in callback item_OnPlace():
/usr/local/share/minetest/builtin/game/item.lua:278: attempt to call global 'check_attached_node' (a nil value)

The issue was a regression of commit 649448a2

"Rename nodeupdate and nodeupdate_single and make them part of the official API"