Misc cleanups and performance improvements.

- There's no need to check rval for prop_dictionary_get_*, we are sure the
  objects are there at prop_dictionary_set_* time.
- Avoid two chdir(2) calls per INSTALL/REMOVE run.
- Avoid using access(2) to check for existence of INSTALL/REMOVE scripts,
  just try to run the executable directly and check for ENOENT.

bin/xbps-repo/util.c

@@ -164,11 +164,7 @@ show_pkg_files(prop_dictionary_t filesd)
 			return EINVAL;
 
 		while ((obj = prop_object_iterator_next(iter))) {
-			if (!prop_dictionary_get_cstring_nocopy(obj,
-			    "file", &file)) {
-				prop_object_iterator_release(iter);
-				return errno;
-			}
+			prop_dictionary_get_cstring_nocopy(obj, "file", &file);
 			printf("%s\n", file);
 		}
 		prop_object_iterator_release(iter);