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/manager.h

@@ -170,6 +170,8 @@ struct Manager {
         int time_change_fd;
         sd_event_source *time_change_event_source;
 
+        sd_event_source *timezone_change_event_source;
+
         sd_event_source *jobs_in_progress_event_source;
 
         int user_lookup_fds[2];
@@ -250,6 +252,10 @@ struct Manager {
 
         unsigned gc_marker;
 
+        /* The stat() data the last time we saw /etc/localtime */
+        usec_t etc_localtime_mtime;
+        bool etc_localtime_accessible:1;
+
         /* Flags */
         ManagerExitCode exit_code:5;