Rename tls13_prepare_* to tls13_add_*.

The SSL code suffers from needing too many verbs for variations on
writing things without actually writing them. We used to have queuing
the message up to be written to the buffer BIO, writing to the buffer
BIO, and flushing the buffer BIO. (Reading, conversely, has a similar
mess of verbs.)

Now we just have adding to the pending flight and flushing the pending
flight, match the SSL_PROTOCOL_METHOD naming.

BUG=72

Change-Id: I332966928bf13f03dfb8eddd519c2fefdd7f24d4
Reviewed-on: https://boringssl-review.googlesource.com/13227
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/ssl/tls13_both.c b/ssl/tls13_both.c
index 6dfc39e..4b44f39 100644
--- a/ssl/tls13_both.c
+++ b/ssl/tls13_both.c
@@ -433,7 +433,7 @@
   return 1;
 }
 
-int tls13_prepare_certificate(SSL_HANDSHAKE *hs) {
+int tls13_add_certificate(SSL_HANDSHAKE *hs) {
   SSL *const ssl = hs->ssl;
   CBB cbb, body, certificate_list;
   if (!ssl->method->init_message(ssl, &cbb, &body, SSL3_MT_CERTIFICATE) ||
@@ -510,8 +510,8 @@
   return 0;
 }
 
-enum ssl_private_key_result_t tls13_prepare_certificate_verify(
-    SSL_HANDSHAKE *hs, int is_first_run) {
+enum ssl_private_key_result_t tls13_add_certificate_verify(SSL_HANDSHAKE *hs,
+                                                           int is_first_run) {
   SSL *const ssl = hs->ssl;
   enum ssl_private_key_result_t ret = ssl_private_key_failure;
   uint8_t *msg = NULL;
@@ -573,7 +573,7 @@
   return ret;
 }
 
-int tls13_prepare_finished(SSL_HANDSHAKE *hs) {
+int tls13_add_finished(SSL_HANDSHAKE *hs) {
   SSL *const ssl = hs->ssl;
   size_t verify_data_len;
   uint8_t verify_data[EVP_MAX_MD_SIZE];