Clearer handling of gray lists when entering generational mode

When entering generational mode, all objects are old. So, the only
objects that need to be in a gray list are threads, which can be
assigned without barriers. Changes in anything else (e.g., weak
tables) will trigger barriers that, if needed, will add the object
to a gray list.

ltests.h

@@ -51,6 +51,7 @@
 
 /* memory-allocator control variables */
 typedef struct Memcontrol {
+  int failnext;
   unsigned long numblocks;
   unsigned long total;
   unsigned long maxmem;