makefs(1): use all the random(3) range.

The generation number is uint32_t so we can fit the complete range
of random(3). We could have used arc4random() but the result would
be unpredictable and it would prohibit reproducible builds.

While here add a comment where seeding is done: this affects
reproducible builds and might have to be re-visited to use a
release dependent value.

MFC after: 2 weeks