DIX: Add strncasecmp from FreeBSD, make strcasecmp args const

Add strncasecmp (as we're now using it) in case someone doesn't have it,
and also change strncasecmp args to be const, in accordance with
everything else.

include/dix.h

@@ -601,7 +601,12 @@ typedef struct {
 /* strcasecmp.c */
 #if NEED_STRCASECMP
 #define strcasecmp xstrcasecmp
-extern int xstrcasecmp(char *s1, char *s2);
+extern int xstrcasecmp(const char *s1, const char *s2);
+#endif
+
+#if NEED_STRNCASECMP
+#define strncasecmp xstrncasecmp
+extern int xstrncasecmp(const char *s1, const char *s2, size_t n);
 #endif
 
 extern int XItoCoreType(int xi_type);