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.

ldo.c

@@ -38,6 +38,19 @@
 #define errorstatus(s)	((s) > LUA_YIELD)
 
 
+/*
+** these macros allow user-specific actions when a thread is
+** resumed/yielded.
+*/
+#if !defined(luai_userstateresume)
+#define luai_userstateresume(L,n)	((void)L)
+#endif
+
+#if !defined(luai_userstateyield)
+#define luai_userstateyield(L,n)	((void)L)
+#endif
+
+
 /*
 ** {======================================================
 ** Error-recovery functions