Function luaD_precall

** Prepares the call to a function (C or Lua). For C functions, also do ** the call. The function to be called is at '*func'. The arguments ** are on the stack, right after the function. Returns the CallInfo ** to be executed, if it was a Lua function. Otherwise (a C function) ** returns NULL, with all the results on the stack, starting at the ** original function position. */