pools: Revert back to "Pool N" default pool name, and use json_array_index to set it.

Closes https://github.com/veox/sgminer/issues/51

Using j_a_i allows us to get rid of accounting vars like total_poolnames,
and pool names don't get mixed up if they're not specified for some pools.

It would still be desirable to show the URL as a pool name if poolname is not
set.