A test failure occurred where a send stream had only a BEGIN record. This
should not be possible if the send returns without error. Prevented this from
happening in the future by adding an assertion to dmu_send_impl() to verify
that if the function returns 0 (success) both a BEGIN and END record are
present. Did this by adding flags to dmu_sendarg_t (indicating whether BEGIN or
END records sent), having dump_record() set flags appropriately, adding VERIFY
statement to dmu_send_impl().