Document why dircache is not included in --list-modules

configure.ac

@@ -290,7 +290,7 @@ AM_COND_IF(BUILD_MODULE_RELAY,
                      [Compile with Relay feature support]))
 
 dnl Dircache module.  (This cannot be enabled or disabled independently of
-dnl the relay module.)
+dnl the relay module.  It is not listed by --list-modules for this reason.)
 AM_CONDITIONAL(BUILD_MODULE_DIRCACHE,
                [test "x$enable_module_relay" != "xno"])
 AM_COND_IF(BUILD_MODULE_DIRCACHE,

src/app/config/config.c

@@ -2733,6 +2733,9 @@ list_enabled_modules(void)
 {
   printf("%s: %s\n", "relay", have_module_relay() ? "yes" : "no");
   printf("%s: %s\n", "dirauth", have_module_dirauth() ? "yes" : "no");
+  // We don't list dircache, because it cannot be enabled or disabled
+  // independently from relay.  Listing it here would proliferate
+  // test variants in test_parseconf.sh to no useful purpose.
 }
 
 /** Last value actually set by resolve_my_address. */