su: Drop "tty" pledge promise after getting password from user

There's not much work left to do at this point, but let's be strict.

Userland/su.cpp

@@ -85,6 +85,11 @@ int main(int argc, char** argv)
  }
  }
 
+ if (pledge("stdio exec id", nullptr) < 0) {
+ perror("pledge");
+ return 1;
+ }
+
  if (!account.login()) {
  perror("Core::Account::login");
  return 1;