main: when reloading PID 1 let's reset the default environment

Otherwise we keep collecting stuff from env generators, and we really
shouldn't.

This was working properly on reexec but not on reload, as for reexec we
would always start fresh, but for reload would reuse the Manager object
and hence its default environment set.

Fixes: #10671

src/core/main.c

@@ -767,8 +767,10 @@ static void set_manager_defaults(Manager *m) {
         m->default_tasks_accounting = arg_default_tasks_accounting;
         m->default_tasks_max = arg_default_tasks_max;
 
-        manager_set_default_rlimits(m, arg_default_rlimit);
-        manager_transient_environment_add(m, arg_default_environment);
+        (void) manager_set_default_rlimits(m, arg_default_rlimit);
+
+        (void) manager_default_environment(m);
+        (void) manager_transient_environment_add(m, arg_default_environment);
 }
 
 static void set_manager_settings(Manager *m) {

src/core/manager.c

@@ -600,7 +600,7 @@ static char** sanitize_environment(char **l) {
         return l;
 }
 
-static int manager_default_environment(Manager *m) {
+int manager_default_environment(Manager *m) {
         assert(m);
 
         m->transient_environment = strv_free(m->transient_environment);

src/core/manager.h

@@ -442,6 +442,7 @@ void manager_clear_jobs(Manager *m);
 
 unsigned manager_dispatch_load_queue(Manager *m);
 
+int manager_default_environment(Manager *m);
 int manager_transient_environment_add(Manager *m, char **plus);
 int manager_client_environment_modify(Manager *m, char **minus, char **plus);
 int manager_get_effective_environment(Manager *m, char ***ret);