coap_session.h: Move coap_new_client_session_pki() def from coap_dtls.h
coap_session.h: Move coap_new_client_session_pki() def from coap_dtls.h

Moving coap_dtls_security_setup_t into coap_dtls_pki_t helped with the previous
into its logical place.

authmillenon/libcoap,authmillenon/libcoap,authmillenon/libcoap