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).

diff --cc src/login/logind-dbus.c
Simple merge

diff --cc src/login/logind-user.c
Simple merge

diff --cc src/login/logind-user.h
Simple merge

diff --cc src/test/meson.build
index 404e366861a6c5ca50265e2d91605b3ccdbdf644,8a25cc03dc35db269f6bb73c5c9af30da797f649..490b9b7d0dcde4c05578d2dd7621eb0d638b0de7
--- 1/src/test/meson.build
--- 2/src/test/meson.build
+++ b/src/test/meson.build
@@@ -493,8 -493,11 +493,12 @@@ executables += 
          test_template + {
                  'sources' : files('test-varlink-idl-machine.c'),
                  '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',