Fix potential race condition in HSTS
Created by: juaristi
Same as f5a63e31006ed02f5de4bb8d947bc7b755bb81fe in wget.
I have seen no file locking in the HSTS code. While it's true it creates some (one?) mutexes, they seem to be for internal threads synchronization only (i.e. to make the HSTS API thread-safe).
Although I haven't had time to prove this yet, it looks like other wget2 processes might clobber the HSTS file.