Update libcurl

Engine/lib/curl/docs/MQTT.md

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+# MQTT in curl
+
+## Usage
+
+A plain "GET" subscribes to the topic and prints all published messages.
+Doing a "POST" publishes the post data to the topic and exits.
+
+Example subscribe:
+
+    curl mqtt://host/home/bedroom/temp
+
+Example publish:
+
+    curl -d 75 mqtt://host/home/bedroom/dimmer
+
+## What does curl deliver as a response to a subscribe
+
+It outputs two bytes topic length (MSB | LSB), the topic followed by the
+payload.
+
+## Caveats
+
+Remaining limitations:
+ - Only QoS level 0 is implemented for publish
+ - No way to set retain flag for publish
+ - No TLS (mqtts) support
+ - Naive EAGAIN handling will not handle split messages