TLS for the MQTT connection (iteration 2, step 5)

ca_file or the system's CAs (MOSQ_OPT_TLS_USE_OS_CERTS), client
certificate, broker certificate and host name verified, settings errors
reported before connecting. The client thread blocks SIGPIPE: with TLS,
a write to a connection the broker closed killed the process. The
reason of a failed handshake (libmosquitto's log) is appended to the
error. tst_mqtt covers TLS with test certificates in tests/data (D44).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/data/client.crt

@@ -0,0 +1,11 @@
+-----BEGIN CERTIFICATE-----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+-----END CERTIFICATE-----