New control for reentrancy of emergency collections

Instead of assuming that shrinking a block may be an emergency
collection, use an explicit field ('gcstopem') to stop emergency
collections while GC is working.

lstate.h

@@ -260,6 +260,7 @@ typedef struct global_State {
   lu_byte currentwhite;
   lu_byte gcstate;  /* state of garbage collector */
   lu_byte gckind;  /* kind of GC running */
+  lu_byte gcstopem;  /* stops emergency collections */
   lu_byte genminormul;  /* control for minor generational collections */
   lu_byte genmajormul;  /* control for major generational collections */
   lu_byte gcrunning;  /* true if GC is running */