logind: migrate to Varlink (phase1) (#41561)

This implements phase 1 of #41560.

Add List Varlink methods for all logind object types (sessions, users,
seats, inhibitors).

src/test/meson.build

@@ -495,6 +495,10 @@ executables += [
                 'objects' : ['systemd-machined'],
                 'conditions' : ['ENABLE_MACHINED'],
         },
+        test_template + {
+                'sources' : files('test-varlink-idl-login.c'),
+                'objects' : ['systemd-logind'],
+        },
         test_template + {
                 'sources' : files('test-watchdog.c'),
                 'type' : 'unsafe',

src/test/test-varlink-idl-login.c

@@ -0,0 +1,21 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+
+#include "logind-inhibit.h"
+#include "logind-session.h"
+#include "tests.h"
+#include "test-varlink-idl-util.h"
+#include "varlink-io.systemd.Login.h"
+
+TEST(login_enums_idl) {
+        TEST_IDL_ENUM(SessionType, session_type, vl_type_SessionType);
+        /* SessionClass has SESSION_NONE ("none") as an internal sentinel that is intentionally not exposed
+         * via Varlink. Only validate the IDL→C direction. */
+        TEST_IDL_ENUM_FROM_STRING(SessionClass, session_class, vl_type_SessionClass);
+        TEST_IDL_ENUM(InhibitMode, inhibit_mode, vl_type_InhibitMode);
+        /* InhibitWhat is a bit-flag enum: inhibit_what_to_string() accepts a bitmask and
+         * the index does not correspond to a sequential integer, so the TEST_IDL_ENUM
+         * round-trip macro doesn't apply. The flag→string mapping is exercised through
+         * ListInhibitors integration tests in TEST-35-LOGIN.sh. */
+}
+
+DEFINE_TEST_MAIN(LOG_DEBUG);