core: serialize and deserialize unit start ratelimits

The logic is taken from dump ratelimit: if the config changes, we discard the
counters. This allows the user apply new limits and "start from scratch" in
that case.

This actually makes StartLimitIntervalSec=infinity (or with a large interval)
work as expected, because the counter is maintained even if daemon-reload
operations are interleaved.

src/core/unit-serialize.c

@@ -140,6 +140,8 @@ int unit_serialize_state(Unit *u, FILE *f, FDSet *fds, bool switching_root) {
         (void) serialize_dual_timestamp(f, "condition-timestamp", &u->condition_timestamp);
         (void) serialize_dual_timestamp(f, "assert-timestamp", &u->assert_timestamp);
 
+        (void) serialize_ratelimit(f, "start-ratelimit", &u->start_ratelimit);
+
         if (dual_timestamp_is_set(&u->condition_timestamp))
                 (void) serialize_bool(f, "condition-result", u->condition_result);
 
@@ -343,6 +345,10 @@ int unit_deserialize_state(Unit *u, FILE *f, FDSet *fds) {
                         (void) deserialize_dual_timestamp(v, &u->assert_timestamp);
                         continue;
 
+                } else if (streq(l, "start-ratelimit")) {
+                        deserialize_ratelimit(&u->start_ratelimit, l, v);
+                        continue;
+
                 } else if (MATCH_DESERIALIZE("condition-result", l, v, parse_boolean, u->condition_result))
                         continue;