esp_netif_init() returns standard esp_err_t error code (unlike tcpip_adapter init), so shall be checked for the return value
Also to make the initialization code more consistent.
The MQTT broker URL used as default in the examples has ceased operation. All examples and documention have been updated to point to the new domain mqtt.eclipse.org.
This also required an update of the python example test scripts to use TLS 1.2
1. add support for ESP32 DTE
2. add support for SIM800/BG96 DCE
3. add PPPoS setup procedure
4. add support for SMS
5. add mqtt example after PPP connection established