logind: register PAM sessions via Varlink instead of D-Bus (#35264)

This makes things a bit faster (because it cuts down a bit on
roundtrips) and prepares ground so that one day we can let logind run in
earlier boot already, making it a bit less special.

communication between logind and pid1 is still dbus only, hence there's
a lot of room for further improvement I guess.

src/test/test-varlink-idl.c

@@ -15,6 +15,7 @@
 #include "varlink-io.systemd.Credentials.h"
 #include "varlink-io.systemd.Import.h"
 #include "varlink-io.systemd.Journal.h"
+#include "varlink-io.systemd.Login.h"
 #include "varlink-io.systemd.Machine.h"
 #include "varlink-io.systemd.MachineImage.h"
 #include "varlink-io.systemd.ManagedOOM.h"
@@ -200,6 +201,8 @@ TEST(parse_format) {
         print_separator();
         test_parse_format_one(&vl_interface_io_systemd_Udev);
         print_separator();
+        test_parse_format_one(&vl_interface_io_systemd_Login);
+        print_separator();
         test_parse_format_one(&vl_interface_xyz_test);
 }