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/core/hashtable.c

@@ -711,7 +711,7 @@ hashtable_iterator_free(HASHITERATOR *iter)
  * @return              Number of entries written or -1 on error
  */
 int
-hashtable_save(HASHTABLE *table, char *filename,
+hashtable_save(HASHTABLE *table, const char *filename,
                int (*keywrite)(int, void*),
                int (*valuewrite)(int, void*))
 {
@@ -771,7 +771,7 @@ hashtable_save(HASHTABLE *table, char *filename,
  * @return              Number of entries read or -1 on error
  */
 int
-hashtable_load(HASHTABLE *table, char *filename,
+hashtable_load(HASHTABLE *table, const char *filename,
                void *(*keyread)(int),
                void *(*valueread)(int))
 {

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));