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.

bin/xbps-repo/show.c

@@ -108,7 +108,7 @@ show_pkg_namedesc(struct xbps_handle *xhp,
 	(void)loop_done;
 
 	prop_dictionary_get_cstring_nocopy(obj, "architecture", &arch);
-	if (!xbps_pkg_arch_match(arch, NULL))
+	if (!xbps_pkg_arch_match(xhp, arch, NULL))
 		return 0;
 
 	prop_dictionary_get_cstring_nocopy(obj, "pkgname", &pkgname);