ratelimit: fix off-by-one

The current code might make sense during startup for service restart
limiting:
With burst=1 after starting num is 1, so the next ('first') restart is
still accepted ( 1 <= 1). However, once interval has expired, num is 1
after the first restart, so the second restart is also accepted.

This change is also useful in combination with watchdogs:
With burst=1 and a very large interval the system can be restarted on
the first failure.