coap_dtls.h: Add support for setting up PKI
coap_dtls.h: Add support for setting up PKI

Define
coap_dtls_pki_t
coap_dtls_security_setup_t

Functions
coap_new_client_session_pki
coap_dtls_context_set_psk
coap_dtls_context_set_pki
coap_dtls_context_check_keys_enabled

authmillenon/libcoap,authmillenon/libcoap,authmillenon/libcoap