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/shared/switch-root.c

@@ -51,7 +51,7 @@ int switch_root(const char *new_root, const char *oldroot, bool detach_oldroot,
         if (path_equal(new_root, "/"))
                 return 0;
 
-        temporary_old_root = strappenda(new_root, oldroot);
+        temporary_old_root = strjoina(new_root, oldroot);
         mkdir_p_label(temporary_old_root, 0755);
 
         old_root_remove = in_initrd();