Rename ssl_conf_datagram_packing() to ssl_set_datagram_packing()

The naming convention is that functions of the form mbedtls_ssl_conf_xxx()
apply to the SSL configuration.
diff --git a/programs/ssl/ssl_server2.c b/programs/ssl/ssl_server2.c
index 8d41436..070c005 100644
--- a/programs/ssl/ssl_server2.c
+++ b/programs/ssl/ssl_server2.c
@@ -2182,7 +2182,7 @@
         mbedtls_ssl_conf_handshake_timeout( &conf, opt.hs_to_min, opt.hs_to_max );
 
     if( opt.dgram_packing != DFL_DGRAM_PACKING )
-        mbedtls_ssl_conf_datagram_packing( &ssl, opt.dgram_packing );
+        mbedtls_ssl_set_datagram_packing( &ssl, opt.dgram_packing );
 #endif /* MBEDTLS_SSL_PROTO_DTLS */
 
 #if defined(MBEDTLS_SSL_MAX_FRAGMENT_LENGTH)