Uphold/StopWhenUnneeded/BindsTo: add retry timer on rate limit

The Upholds= promise is that as long as unit A is up and Upholds=B,
B will be activated if failed or inactive. But there is a hard-coded,
non-configurable rate limit for this, so add a timed retry after the
ratelimit has expired.

Apply to BindsTo= and StopWhenUnneeded= as well.

src/basic/ratelimit.c

@@ -38,3 +38,12 @@ unsigned ratelimit_num_dropped(RateLimit *r) {
 
         return r->num > r->burst ? r->num - r->burst : 0;
 }
+
+usec_t ratelimit_end(const RateLimit *rl) {
+        assert(rl);
+
+        if (rl->begin == 0)
+                return 0;
+
+        return usec_add(rl->begin, rl->interval);
+}

src/basic/ratelimit.h

@@ -23,3 +23,5 @@ static inline bool ratelimit_configured(RateLimit *rl) {
 bool ratelimit_below(RateLimit *r);
 
 unsigned ratelimit_num_dropped(RateLimit *r);
+
+usec_t ratelimit_end(const RateLimit *rl);