coap_session.c: Support PSK new function and PKI
coap_session.c: Support PSK new function and PKI

Update coap_new_client_session_psk() to clean up and return on error.  Add in
call to coap_dtls_context_set_psk()

Add in new function coap_new_client_session_pki()

Additional check for a call of coap_new_client_session_*() in
coap_new_endpoint()

authmillenon/libcoap,authmillenon/libcoap,authmillenon/libcoap