spooldir mtime=0 prevents database from being updated

When the spooldir mtime=0, cron refuses to update its "database". This happens because the "old database" mtime is initialized to 0.

```
61: /* if spooldir's mtime has not changed, we don't need to fiddle with
62- * the database.
63- *
64- * Note that old_db->mtime is initialized to 0 in main(), and
65- * so is guaranteed to be different than the stat() mtime the first
66- * time this function is called.
67- */
68- if (old_db->mtime == TMAX(statbuf.st_mtime, syscron_stat.st_mtime)) {
69- Debug(DLOAD, ("[%ld] spool dir mtime unch, no load needed.\n",
70- (long)getpid()))
71- return;
72- }
```

from `database.c`, v4.1