util: rework strappenda(), and rename it strjoina()

After all it is now much more like strjoin() than strappend(). At the
same time, add support for NULL sentinels, even if they are normally not
necessary.

src/run/run.c

@@ -494,7 +494,7 @@ static int transient_service_set_properties(sd_bus_message *m, char **argv, cons
                 if (e) {
                         char *n;
 
-                        n = strappenda("TERM=", e);
+                        n = strjoina("TERM=", e);
                         r = sd_bus_message_append(m,
                                                   "(sv)",
                                                   "Environment", "as", 1, n);