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_conflicts.c

@@ -66,7 +66,7 @@ xbps_pkg_find_conflicts(struct xbps_handle *xhp, prop_dictionary_t pkg_repod)
 		/*
 		 * Check if current pkg conflicts with any pkg in transaction.
 		 */
-		pkgd = xbps_find_pkg_in_dict_by_pattern(xhp->transd,
+		pkgd = xbps_find_pkg_in_dict_by_pattern(xhp, xhp->transd,
 		    "unsorted_deps", cfpkg);
 		if (pkgd != NULL) {
 			prop_dictionary_get_cstring_nocopy(pkgd,