Added a warning system to Lua

The warning system is just a way for Lua to emit warnings, messages
to the programmer that do not interfere with the running program.

lstate.h

@@ -231,6 +231,8 @@ typedef struct global_State {
   TString *tmname[TM_N];  /* array with tag-method names */
   struct Table *mt[LUA_NUMTAGS];  /* metatables for basic types */
   TString *strcache[STRCACHE_N][STRCACHE_M];  /* cache for strings in API */
+  lua_WarnFunction warnf;  /* warning function */
+  void *ud_warn;         /* auxiliary data to 'warnf' */
 } global_State;