Changed signal of GC debt

Positive debts seems more natural then negative ones.

llimits.h

@@ -33,6 +33,8 @@ typedef unsigned long lu_mem;
 typedef long l_obj;
 #endif				/* } */
 
+#define MAX_LOBJ	cast(l_obj, ~cast(lu_mem, 0) >> 1)
+
 
 /* chars used as small naturals (so that 'char' is reserved for characters) */
 typedef unsigned char lu_byte;
@@ -47,11 +49,6 @@ typedef signed char ls_byte;
                           : (size_t)(LUA_MAXINTEGER))
 
 
-#define MAX_LUMEM	((lu_mem)(~(lu_mem)0))
-
-#define MAX_LMEM	((l_obj)(MAX_LUMEM >> 1))
-
-
 #define MAX_INT		INT_MAX  /* maximum value of an int */