net: sockets: tls: Return ENOTCONN when DTLS client session ends
net: sockets: tls: Return ENOTCONN when DTLS client session ends

Notify the application when DTLS client session ends by returning
ENOTCONN on such event. Additionally, reset the mbed TLS session
structures, allowing to reinstante the session on the next send() call.

Signed-off-by: Robert Lubos <9d28997c6894a2d0321a7bb399169af7ce99b64e@nordicsemi.no>

zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,galak/zephyr