core: use uniform style for RateLimit initialization

RateLimit is designed so that we can always initialize only the first two
fields explicitly. All other call sites use a single line for this.

src/basic/ratelimit.h

@@ -12,6 +12,8 @@ typedef struct RateLimit {
         usec_t begin;
 } RateLimit;
 
+#define RATELIMIT_OFF (const RateLimit) { .interval = USEC_INFINITY, .burst = UINT_MAX }
+
 static inline void ratelimit_reset(RateLimit *rl) {
         rl->num = rl->begin = 0;
 }