Make foo_validate() functions call config_validate().

The former foo_validate() functions are now toplevel
legacy_validate_fn callbacks.  The new foo_validate() functions now
call them.

This change lets us remove the old shared_random disk state
validation callback entirely.

src/feature/dirauth/shared_random_state.c

@@ -59,9 +59,6 @@ DUMMY_TYPECHECK_INSTANCE(sr_disk_state_t);
 /** Our persistent state magic number. */
 #define SR_DISK_STATE_MAGIC 0x98AB1254
 
-static int
-disk_state_validate_cb(const void *old_state, void *state, char **msg);
-
 /** Array of variables that are saved to disk as a persistent state. */
 static const config_var_t state_vars[] = {
   V(Version,                    POSINT, "0"),
@@ -94,7 +91,6 @@ static const config_format_t state_format = {
    offsetof(sr_disk_state_t, magic_),
   },
   .vars = state_vars,
-  .legacy_validate_fn = disk_state_validate_cb,
   .extra = &state_extra_var,
   .config_suite_offset = -1,
 };
@@ -339,21 +335,6 @@ disk_state_validate(const sr_disk_state_t *state)
   return -1;
 }
 
-/** Validate the disk state (NOP for now). */
-static int
-disk_state_validate_cb(const void *old_state, void *state, char **msg)
-{
-  /* We don't use these; only options do. */
-  (void) old_state;
-
-  /* This is called by config_dump which is just before we are about to
-   * write it to disk. At that point, our global memory state has been
-   * copied to the disk state so it's fair to assume it's trustable. */
-  (void) state;
-  (void) msg;
-  return 0;
-}
-
 /** Parse the Commit line(s) in the disk state and translate them to the
  * the memory state. Return 0 on success else -1 on error. */
 static int