Don't use the buffer BIO in TLS.

On the TLS side, we introduce a running buffer of ciphertext. Queuing up
pending data consists of encrypting the record into the buffer. This
effectively reimplements what the buffer BIO was doing previously, but
this resizes to fit the whole flight.

As part of this, rename all the functions to add to the pending flight
to be more uniform. This CL proposes "add_foo" to add to the pending
flight and "flush_flight" to drain it.

We add an add_alert hook for alerts but, for now, only the SSL 3.0
warning alert (sent mid-handshake) uses this mechanism.  Later work will
push this down to the rest of the write path so closure alerts use it
too, as in DTLS. The intended end state is that all the ssl_buffer.c and
wpend_ret logic will only be used for application data and eventually
optionally replaced by the in-place API, while all "incidental" data
will be handled internally.

For now, the two buffers are mutually exclusive. Moving closure alerts
to "incidentals" will change this, but flushing application data early
is tricky due to wpend_ret. (If we call ssl_write_buffer_flush,
do_ssl3_write doesn't realize it still has a wpend_ret to replay.) That
too is all left alone in this change.

To keep the diff down, write_message is retained for now and will be
removed from the state machines in a follow-up change.

BUG=72

Change-Id: Ibce882f5f7196880648f25d5005322ca4055c71d
Reviewed-on: https://boringssl-review.googlesource.com/13224
Reviewed-by: Adam Langley <agl@google.com>

ssl/d1_both.c

diff --git a/ssl/d1_both.c b/ssl/d1_both.c
index 1d89636..bbb6b32 100644
--- a/ssl/d1_both.c
+++ b/ssl/d1_both.c
@@ -579,7 +579,7 @@
   return 1;
 }
 
-int dtls1_queue_message(SSL *ssl, uint8_t *data, size_t len) {
+int dtls1_add_message(SSL *ssl, uint8_t *data, size_t len) {
   return add_outgoing(ssl, 0 /* handshake */, data, len);
 }
 
@@ -588,10 +588,19 @@
   return 1;
 }
 
-int dtls1_send_change_cipher_spec(SSL *ssl) {
+int dtls1_add_change_cipher_spec(SSL *ssl) {
   return add_outgoing(ssl, 1 /* ChangeCipherSpec */, NULL, 0);
 }
 
+int dtls1_add_alert(SSL *ssl, uint8_t level, uint8_t desc) {
+  /* The |add_alert| path is only used for warning alerts for now, which DTLS
+   * never sends. This will be implemented later once closure alerts are
+   * converted. */
+  assert(0);
+  OPENSSL_PUT_ERROR(SSL, ERR_R_INTERNAL_ERROR);
+  return 0;
+}
+
 /* dtls1_update_mtu updates the current MTU from the BIO, ensuring it is above
  * the minimum. */
 static void dtls1_update_mtu(SSL *ssl) {