core: subscribe to /etc/localtime timezone changes and update timer elapsation accordingly

Fixes: #8233

This is our first real-life usecase for the new sd_event_add_inotify()
calls we just added.

src/core/timer.c

@@ -819,6 +819,18 @@ static void timer_time_change(Unit *u) {
         timer_enter_waiting(t, false);
 }
 
+static void timer_timezone_change(Unit *u) {
+        Timer *t = TIMER(u);
+
+        assert(u);
+
+        if (t->state != TIMER_WAITING)
+                return;
+
+        log_unit_debug(u, "Timezone change, recalculating next elapse.");
+        timer_enter_waiting(t, false);
+}
+
 static const char* const timer_base_table[_TIMER_BASE_MAX] = {
         [TIMER_ACTIVE] = "OnActiveSec",
         [TIMER_BOOT] = "OnBootSec",
@@ -868,6 +880,7 @@ const UnitVTable timer_vtable = {
 
         .reset_failed = timer_reset_failed,
         .time_change = timer_time_change,
+        .timezone_change = timer_timezone_change,
 
         .bus_vtable = bus_timer_vtable,
         .bus_set_property = bus_timer_set_property,