passwd: Prompt for the current password before setting new password

This changes passwd to authenticate non-root users before prompting for
new password.

Userland/Utilities/passwd.cpp

@@ -84,6 +84,19 @@ int main(int argc, char** argv)
  } else if (unlock) {
  target_account.set_password_enabled(true);
  } else {
+ if (current_uid != 0) {
+ auto current_password = Core::get_password("Current password: ");
+ if (current_password.is_error()) {
+ warnln("{}", current_password.error());
+ return 1;
+ }
+
+ if (!target_account.authenticate(current_password.value().characters())) {
+ warnln("Incorrect or disabled password.");
+ return 1;
+ }
+ }
+
  auto new_password = Core::get_password("New password: ");
  if (new_password.is_error()) {
  warnln("{}", new_password.error());