Mark all SSL_CIPHERs as const.

This lets us put the SSL_CIPHER table in the data section. For type-checking,
make STACK_OF(SSL_CIPHER) cast everything to const SSL_CIPHER*.

Note that this will require some changes in consumers which weren't using a
const SSL_CIPHER *.

Change-Id: Iff734ac0e36f9e5c4a0f3c8411c7f727b820469c
Reviewed-on: https://boringssl-review.googlesource.com/1541
Reviewed-by: Adam Langley <agl@google.com>
diff --git a/ssl/s3_srvr.c b/ssl/s3_srvr.c
index 6a028ef..19a121d 100644
--- a/ssl/s3_srvr.c
+++ b/ssl/s3_srvr.c
@@ -821,7 +821,7 @@
 	{
 	int i,ok,al=SSL_AD_INTERNAL_ERROR,ret= -1;
 	long n;
-	SSL_CIPHER *c;
+	const SSL_CIPHER *c;
 	STACK_OF(SSL_CIPHER) *ciphers=NULL;
 	struct ssl_early_callback_ctx early_ctx;
 	CBS client_hello;
@@ -1152,7 +1152,7 @@
 
 	if (!s->hit && s->version >= TLS1_VERSION && s->tls_session_secret_cb)
 		{
-		SSL_CIPHER *pref_cipher=NULL;
+		const SSL_CIPHER *pref_cipher=NULL;
 
 		s->session->master_key_length=sizeof(s->session->master_key);
 		if(s->tls_session_secret_cb(s, s->session->master_key, &s->session->master_key_length,