Make hashtable const correct.

Hashtable need not modify the filename when loading or saving
the hashtable; consequently the filename can be passed as const.

server/include/hashtable.h

@@ -119,11 +119,11 @@ void hashtable_get_stats(void* hashtable,
                          int*  nelems,
                          int*  longest);
 extern int hashtable_save(HASHTABLE *,
-                          char *,
+                          const char *filename,
                           int (*keywrite)(int, void*),
                           int (*valuewrite)(int, void*));
 extern int hashtable_load(HASHTABLE *,
-                          char *,
+                          const char *filename,
                           void *(*keyread)(int),
                           void *(*valueread)(int));