tree-wide: say "ratelimit" not "rate_limit"

"ratelimit" is a real word, so we don't need to use the other form anywhere.
We had both forms in various places, let's standarize on the shorter and more
correct one.

src/core/execute.h

@@ -245,8 +245,8 @@ struct ExecContext {
         struct iovec* log_extra_fields;
         size_t n_log_extra_fields;
 
-        usec_t log_rate_limit_interval_usec;
-        unsigned log_rate_limit_burst;
+        usec_t log_ratelimit_interval_usec;
+        unsigned log_ratelimit_burst;
 
         int log_level_max;