curl_easy_setopt(). The bitmask has two separate flags that can be used, but
CURLHSTS_ENABLEis the primary one. If that is set, then this easy handle how has HSTS support enabled.
CURLHSTS_READONLYFILE, which if set, tells libcurl that the file name you specify for it to use as a HSTS cache is only to be read from, and not write anything back to.
CURLOPT_HSTSoption. libcurl will read from this file at start of a transfer and write to it (unless it was set read-only) when the easy handle is closed.