core: limit timestamp to sane precision

Anything below .1 s is meaningless anyway.

src/core/main.c

@@ -1725,7 +1725,7 @@ int main(int argc, char *argv[]) {
                 after_startup = now(CLOCK_MONOTONIC);
                 log_full(arg_action == ACTION_TEST ? LOG_INFO : LOG_DEBUG,
                          "Loaded units and determined initial transaction in %s.",
-                         format_timespan(timespan, sizeof(timespan), after_startup - before_startup, 0));
+                         format_timespan(timespan, sizeof(timespan), after_startup - before_startup, 100 * USEC_PER_MSEC));
 
                 if (arg_action == ACTION_TEST) {
                         printf("-> By jobs:\n");