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/transaction_package_replace.c

@@ -108,7 +108,7 @@ xbps_transaction_package_replace(struct xbps_handle *xhp)
 			 * transaction and it's going to be updated.
 			 */
 			reppkgd = xbps_find_pkg_in_array_by_name(
-			    transd_unsorted, curpkgname, NULL);
+			    xhp, transd_unsorted, curpkgname, NULL);
 			if (reppkgd) {
 				xbps_dbg_printf(xhp,
 				    "found replaced pkg "