Cleaning of llimits.h

Several definitions that don't need to be "global" (that is, that
concerns only specific parts of the code) moved out of llimits.h,
to more appropriate places.

lstate.c

@@ -51,6 +51,27 @@ typedef struct LG {
 #define fromstate(L)	(cast(LX *, cast(lu_byte *, (L)) - offsetof(LX, l)))
 
 
+/*
+** these macros allow user-specific actions when a thread is
+** created/deleted
+*/
+#if !defined(luai_userstateopen)
+#define luai_userstateopen(L)		((void)L)
+#endif
+
+#if !defined(luai_userstateclose)
+#define luai_userstateclose(L)		((void)L)
+#endif
+
+#if !defined(luai_userstatethread)
+#define luai_userstatethread(L,L1)	((void)L)
+#endif
+
+#if !defined(luai_userstatefree)
+#define luai_userstatefree(L,L1)	((void)L)
+#endif
+
+
 /*
 ** set GCdebt to a new value keeping the real number of allocated
 ** objects (totalobjs - GCdebt) invariant and avoiding overflows in