Improve snapshot config flexibility

[tarkhil]

Since ZFS allows to store arbitrary attributes, maybe it would be useful to store number of snapshots to keep in variables like org.freebsd:snapshots.hourly|daily|weekly|monhtly?

I'll make a patch if someone is interested

[ankerstal]

So you can have different number of stored snapshots for every filesystem? Sounds reasonable. But I think default values should still be read from periodic.conf.