Add mbedtls_ssl_set_hs_authmode

While at it, fix the following:
- on server with RSA_PSK, we don't want to set flags (client auth happens via
  the PSK, no cert is expected).
- use safer tests (eg == OPTIONAL vs != REQUIRED)

include/mbedtls/ssl_internal.h

@@ -183,6 +183,7 @@ struct mbedtls_ssl_handshake_params
 #if defined(MBEDTLS_X509_CRT_PARSE_C)
     mbedtls_ssl_key_cert *key_cert;     /*!< chosen key/cert pair (server)  */
 #if defined(MBEDTLS_SSL_SERVER_NAME_INDICATION)
+    int sni_authmode;                   /*!< authmode from SNI callback     */
     mbedtls_ssl_key_cert *sni_key_cert; /*!< key/cert list from SNI         */
     mbedtls_x509_crt *sni_ca_chain;     /*!< trusted CAs from SNI callback  */
     mbedtls_x509_crl *sni_ca_crl;       /*!< trusted CAs CRLs from SNI      */