Debug information about extra arguments from __call

'debug.getinfo' can return number of extra arguments added to a call by
a chain of __call metavalues. That information is being used to improve
error messages about errors in these extra arguments.

lua.h

@@ -504,6 +504,7 @@ struct lua_Debug {
   unsigned char nups;	/* (u) number of upvalues */
   unsigned char nparams;/* (u) number of parameters */
   char isvararg;        /* (u) */
+  unsigned char extraargs;  /* (t) number of extra arguments */
   char istailcall;	/* (t) */
   int ftransfer;   /* (r) index of first value transferred */
   int ntransfer;   /* (r) number of transferred values */