Previous code incorrectly found obsolete files in this case:
(curpkg) - in files: /etc/foo.conf
(newpkg) - in conf_files: /etc/foo.conf
and removed this file because the code couldn't find it in the same
array. The new code fixes this case and compares the whole pkg filelist.
1- We can cache the result of the first xbps_pkgdb_init() when it fails
and avoid the malloc/free/access from it.
2- We cache the uname(2) result into a private var in xbps_handle and
use it in xbps_pkg_arch_match().
This improves performance by ~5% approx and it's close as it was before
introducing the repository index format 1.5.
