digest: flush proxy state on proxy or credential change

[alhudz]

Repro: reuse one easy handle with CURLOPT_PROXYAUTH = CURLAUTH_DIGEST, drive a request through one digest proxy, then point CURLOPT_PROXY at a different proxy (or change CURLOPT_PROXYUSERPWD) and perform again.

Cause: Curl_output_digest() flushes data->state.digest when the host origin or credentials change, but the proxy branch reuses data->state.proxydigest unconditionally, so a stale nonce/nc/realm from the previous proxy is carried into the next Proxy-Authorization. 5c6b488 added the guard for the host branch only.

Fix: mirror that guard in the proxy branch, comparing the stored state against data->conn->http_proxy.peer and data->conn->http_proxy.creds and flushing on a mismatch.

[bagder]

The proxy case is already handled differently, but I believe this take is the better and more robust take. It may however allow us to remove the other logic. See 5c22538, f59733b and c1cfdf5

[alhudz]

Combined the two branches into a shared helper now. On removing the other logic: the setopt.c/url.c paths also reset data->state.authproxy (the picked mechanism), which this guard doesn't touch, so I left them in place here — the Curl_auth_digest_cleanup() calls there do look redundant with this in though. Verified locally: 1588, 1589 and 1686 pass with the combined helper, plus the full HTTP Digest auth keyword set (53/53). 1647 skips for me (no stunnel).

[bagder]

neat!

[bagder]

Thanks!