logind: let system-wide idle begin at the time logind was initialized

Initialize the start of the system-wide idle time with the time logind was
initialized and not with the start of the Unix epoch. This means that systemd
will not repport a unreasonable long idle time (around 54 years at the time of
writing this), especially at in the early boot, while no login manager session,
e.g,. gdm, had a chance to provide a more accurate start of the idle period.

Fixes #35163

(cherry picked from commit 718b31138b)
(cherry picked from commit 9d36809256)

src/login/logind-core.c

@@ -404,10 +404,14 @@ int manager_get_user_by_pid(Manager *m, pid_t pid, User **ret) {
 int manager_get_idle_hint(Manager *m, dual_timestamp *t) {
         Session *s;
         bool idle_hint;
-        dual_timestamp ts = DUAL_TIMESTAMP_NULL;
+        dual_timestamp ts;
 
         assert(m);
 
+        /* Initialize the baseline timestamp with the time the manager got initialized to avoid reporting
+         * unreasonable large idle periods starting with the Unix epoch. */
+        ts = m->init_ts;
+
         idle_hint = !manager_is_inhibited(m, INHIBIT_IDLE, INHIBIT_BLOCK, t, false, false, 0, NULL);
 
         HASHMAP_FOREACH(s, m->sessions) {

src/login/logind.c

@@ -104,6 +104,8 @@ static int manager_new(Manager **ret) {
 
         (void) sd_event_set_watchdog(m->event, true);
 
+        dual_timestamp_now(&m->init_ts);
+
         manager_reset_config(m);
 
         *ret = TAKE_PTR(m);

src/login/logind.h

@@ -141,6 +141,8 @@ struct Manager {
 
         char *efi_loader_entry_one_shot;
         struct stat efi_loader_entry_one_shot_stat;
+
+        dual_timestamp init_ts;
 };
 
 void manager_reset_config(Manager *m);