libxbps: performance improvements by caching the most accessed paths.

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.

lib/package_orphans.c

@@ -137,7 +137,7 @@ find_orphan_pkg(struct xbps_handle *xhp,
 			prop_object_iterator_release(iter);
 			return EINVAL;
 		}
-		if (xbps_find_pkg_in_array_by_pattern(od->array, pkgdep, NULL))
+		if (xbps_find_pkg_in_array_by_pattern(xhp, od->array, pkgdep, NULL))
 			ndep++;
 		if (od->orphans_user == NULL)
 			continue;