Refactor ChaCha20-Poly1305 AEAD nonce handling.

This change reduces unnecessary copying and makes the pre-RFC-7539
nonces 96 bits just like the AES-GCM, AES-CCM, and RFC 7539
ChaCha20-Poly1305 cipher suites. Also, all the symbols related to
the pre-RFC-7539 cipher suites now have "_OLD" appended, in
preparation for adding the RFC 7539 variants.

Change-Id: I1f85bd825b383c3134df0b6214266069ded029ae
Reviewed-on: https://boringssl-review.googlesource.com/6103
Reviewed-by: Adam Langley <alangley@gmail.com>

ssl/internal.h

diff --git a/ssl/internal.h b/ssl/internal.h
index 0a6c31c..76a31bf 100644
--- a/ssl/internal.h
+++ b/ssl/internal.h
@@ -181,7 +181,7 @@
 #define SSL_AES256 0x00000008L
 #define SSL_AES128GCM 0x00000010L
 #define SSL_AES256GCM 0x00000020L
-#define SSL_CHACHA20POLY1305 0x00000040L
+#define SSL_CHACHA20POLY1305_OLD 0x00000040L
 #define SSL_eNULL 0x00000080L
 
 #define SSL_AES (SSL_AES128 | SSL_AES256 | SSL_AES128GCM | SSL_AES256GCM)