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/server.key

@@ -0,0 +1,5 @@
+-----BEGIN PRIVATE KEY-----
+MIGHAgEAMBMGByqGSM49AgEGCCqGSM49AwEHBG0wawIBAQQgP5sIs6digdS0/HXU
+a8G16oZk3HZwfYHTjnAIm0APWpuhRANCAASQglF3Qygu2k/RfvnlluNi9XRWnasM
+QJBnILrhtvC5++Ta+bX4lQG6yeedIh+pFTlLfAfqnNLJbFmK5zVNexyu
+-----END PRIVATE KEY-----