This routine will spawn a thread per core to process N items stored
in the specified array, the last thread gets the remainder of items left.
Results have shown that xbps benefits if there is a considerable amount
of items and number of threads being spawned.
Use it in xbps_pkgdb_foreach_cb(), xbps-pkgdb(8), xbps-query(8)
and xbps-rindex(8).
On UP systems there's no overhead because pthread(3) is not used at all.
WIP! investigate if it can be used in libxbps (xbps_rpool_foreach()),
and finish conversion of xbps-rindex(8) -c.
