mkfs.famfs: support configurable metadata log size?

[jagalactic]

Currently the log is always 8MiB, which currently holds something over 25000 log entries. Reasons we might support a configurable log size:

1. 25K entries is not enough
2. 8MiB is massive overkill (e.g. the device is going to be used for a single file, or a small set). In this case, the minimum log size will be 2MiB - one huge page.

Under no circumstances should we support a log size that is not a 2MiB multiple. Honestly I'm not sure this needs to be addressed, but it's a question that has come up several times.

The trickiest part would be test coverage.