timer: reenable TIMER_ACTIVE timers when restarted

A timer configured with OnActiveSec will start its associated unit again
if the timer is stopped, then started. However, if the timer unit is
restarted -- with "systemctl restart", say -- this does not occur.

This commit ensures that TIMER_ACTIVE timers are re-enabled whenever the
timer is started, even if that's within a restart job.

(cherry picked from commit 779042e772)

src/core/timer.c

@@ -521,6 +521,7 @@ fail:
 
 static int timer_start(Unit *u) {
         Timer *t = TIMER(u);
+        TimerValue *v;
 
         assert(t);
         assert(t->state == TIMER_DEAD || t->state == TIMER_FAILED);
@@ -530,6 +531,11 @@ static int timer_start(Unit *u) {
 
         t->last_trigger = DUAL_TIMESTAMP_NULL;
 
+        /* Reenable all timers that depend on unit activation time */
+        LIST_FOREACH(value, v, t->values)
+                if (v->base == TIMER_ACTIVE)
+                        v->disabled = false;
+
         if (t->stamp_path) {
                 struct stat st;