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.
The list of required external deps is now confuse, libarchive and openssl.
libxbps now includes a wrapper for proplib prefixed with xbps_ rather than prop_.
These are the core interfaces in the new API:
rpool - Interface to interact with the repository pool.
rindex - Interface to interact with repository indexes.
pkgdb - Interface to interact with local packages.
transaction - Interface to interact with a transaction.
This also brings new repository index format, making the index file
per architecture and being incompatible with previous versions.
The transaction frequency flush option has been removed, and due to
the nature of package states it was causing more harm than good.
More changes coming soon, but the API shall remain stable from now on.
