Mongoose 6.9

API changes:

• MG_ENABLE_CALLBACK_USERDATA - if set, changes signature of event handler function to include user_data argument. Disabled by default for now, in the future this will become the default.
• mg_set_nameserver() - specify DNS server to use
• mg_assemble_uri() - assemble a URI from parts
• mg_connect_ws() now accepts http:// URLs

Bug fixes:

• Fix parsing of MQTT QoS > 0 PUBLISH messages
• Fix MQTT PUB{ACK,REC,REL,COMP} and UNSUBACK flags
• Properly shut down the SSL connection (send "close notify" TLS message)
• Fix mg_get_http_var() return value
• Fix MQTT handshake; change client protocol to version 3.1.1
• Fix Handling of multiple MQTT messages per RECV event
• Update to make lwip_net_if thread-safe
• Use DhcpNameServer on Windows
• Fix MQTT message parsing issues
• Fix DNS name uncompression that could lead to infinite loop
• Fix WS frame reassembly issues

#855