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/systemctl/systemctl.c

@@ -4978,8 +4978,8 @@ static int switch_root(sd_bus *bus, char **args) {
         if (init) {
                 const char *root_systemd_path = NULL, *root_init_path = NULL;
 
-                root_systemd_path = strappenda(root, "/" SYSTEMD_BINARY_PATH);
-                root_init_path = strappenda(root, "/", init);
+                root_systemd_path = strjoina(root, "/" SYSTEMD_BINARY_PATH);
+                root_init_path = strjoina(root, "/", init);
 
                 /* If the passed init is actually the same as the
                  * systemd binary, then let's suppress it. */
@@ -5783,7 +5783,7 @@ static int unit_file_create_dropin(const char *unit_name, const char *user_home,
         assert(ret_new_path);
         assert(ret_tmp_path);
 
-        ending = strappenda(unit_name, ".d/override.conf");
+        ending = strjoina(unit_name, ".d/override.conf");
         r = get_file_to_edit(ending, user_home, user_runtime, &tmp_new_path);
         if (r < 0)
                 return r;