Small corrections when setting 'L->top'

- OP_NEWTABLE can use 'ra + 1' to set top (instead of ci->top);
- OP_CLOSE doesn't need to set top ('Protect' already does that);
- OP_TFORCALL must use 'ProtectNT', to preserve the top already set.
  (That was a small bug, because iterators could be called with
   extra parameters besides the state and the control variable.)
- Comments and an extra test for the bug in previous item.

lapi.h

@@ -11,12 +11,22 @@
 #include "llimits.h"
 #include "lstate.h"
 
+
+/* Increments 'L->top', checking for stack overflows */
 #define api_incr_top(L)   {L->top++; api_check(L, L->top <= L->ci->top, \
 				"stack overflow");}
 
+
+/*
+** If a call returns too many multiple returns, the callee may not have
+** stack space to accomodate all results. In this case, this macro
+** increases its stack space ('L->ci->top').
+*/
 #define adjustresults(L,nres) \
     { if ((nres) <= LUA_MULTRET && L->ci->top < L->top) L->ci->top = L->top; }
 
+
+/* Ensure the stack has at least 'n' elements */
 #define api_checknelems(L,n)	api_check(L, (n) < (L->top - L->ci->func), \
 				  "not enough elements in the stack")