Fix basic auth denial for proxy-protocol cert clients

[pieterhollander]

Problem

When a listener uses PROXY protocol v2 with use_identity_as_username true,
clients authenticated via mTLS at the proxy are incorrectly denied by
mosquitto_basic_auth().

With direct TLS, handle_connect.c skips mosquitto_basic_auth() when
ssl_ctx is set and use_identity_as_username is enabled — authentication
is "assumed to be cleared." However, with proxy protocol, ssl_ctx is NULL
(TLS was terminated externally), so the bypass does not trigger. The client
falls through to mosquitto_basic_auth(), all plugins defer (no password),
and the allow_anonymous fallback rejects it because username != NULL.

Code path (before fix)

handle_connect.c:1251
  → ssl_ctx is NULL (proxy protocol, no direct TLS)
  → condition FAILS: ssl_ctx && (use_identity_as_username || ...)
  → falls through to mosquitto_basic_auth()

plugin_basic_auth.c:101
  → all plugins defer (no password to check)
  → anonymous fallback: username == NULL? NO (it's the cert CN)
  → DENIED

Fix

Extend the auth bypass condition to also recognize proxy protocol listeners:

if((context->listener->ssl_ctx || context->listener->enable_proxy_protocol > 0)
        && (context->listener->use_identity_as_username || context->listener->use_subject_as_username)){
    /* Authentication assumed to be cleared */
}

When enable_proxy_protocol > 0 and use_identity_as_username is set, the
client was authenticated externally via certificate — same as direct TLS.

Testing

Tested with:

• Proxy protocol v2 listener + use_identity_as_username true
• mTLS cert clients connecting through an external TLS-terminating proxy
• Confirmed: cert clients connect successfully (CONNACK 0)
• Confirmed: passwordless clients on non-proxy listeners are still denied

It seems the fixes branch doesn't contain Mosquitto 2.1 with PROXY protocol support yet. Therefore, I'm submitting this to "master".

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[pieterhollander]

@ralight I was wondering if perhaps you'd be willing to have a look at this small bug fix PR? I would really appreciate it, as that would allow me to stop using my test branch in my setup. Thanks in advance for your efforts!