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/system-update-generator/system-update-generator.c

@@ -45,7 +45,7 @@ static int generate_symlink(void) {
                 return -EINVAL;
         }
 
-        p = strappenda(arg_dest, "/default.target");
+        p = strjoina(arg_dest, "/default.target");
         if (symlink(SYSTEM_DATA_UNIT_PATH "/system-update.target", p) < 0)
                 return log_error_errno(errno, "Failed to create symlink %s: %m", p);