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-uhelper/main.c

@@ -209,10 +209,7 @@ main(int argc, char **argv)
 		if (dict == NULL)
 			exit(EXIT_FAILURE);
 
-		if (!prop_dictionary_get_cstring_nocopy(dict, "version",
-		    &version))
-			exit(EXIT_FAILURE);
-
+		prop_dictionary_get_cstring_nocopy(dict, "version", &version);
 		printf("%s\n", version);
 		prop_object_release(dict);