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.

lstring.h

@@ -19,6 +19,17 @@
 #define MEMERRMSG       "not enough memory"
 
 
+/*
+** Maximum length for short strings, that is, strings that are
+** internalized. (Cannot be smaller than reserved words or tags for
+** metamethods, as these strings must be internalized;
+** #("function") = 8, #("__newindex") = 10.)
+*/
+#if !defined(LUAI_MAXSHORTLEN)
+#define LUAI_MAXSHORTLEN	40
+#endif
+
+
 /*
 ** Size of a short TString: Size of the header plus space for the string
 ** itself (including final '\0').