• Don't allow an unprivileged user to set the stride.
• Only set the stride under the softc lock.
• Rename the internal fields to accurately reflect their use. I don't understand where the "bucket" terminology comes from. Keep ro_bkt to avoid changing the user API.
• Simplify the implementation. The port index is just sc_seq / stride.
• Document rr_limit in ifconfig.8.

I don't like the ifconfig option name of "rr_limit" as it doesn't really
express what the option means. I could rename that too and keep the old
name aliased for compatibility.