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 718b311)
(cherry picked from commit 9d36809)
(cherry picked from commit 77b963c)
(cherry picked from commit cd6f07e)
(cherry picked from commit dcb86ed)
(cherry picked from commit 331f36c)

Author: Flowdalic

src/login/logind-core.c

@@ -401,10 +401,14 @@ 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

@@ -87,6 +87,8 @@ static int manager_new(Manager **ret) {
 
         (void) sd_event_set_watchdog(m->event, true);
 
+        dual_timestamp_get(&m->init_ts);
+
         manager_reset_config(m);
 
         *ret = TAKE_PTR(m);

src/login/logind.h

@@ -139,6 +139,8 @@ 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);