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>

ssl/t1_lib.c

diff --git a/ssl/t1_lib.c b/ssl/t1_lib.c
index f6518c4..cdbc6c3 100644
--- a/ssl/t1_lib.c
+++ b/ssl/t1_lib.c
@@ -937,7 +937,7 @@
 
 		for (i = 0; i < sk_SSL_CIPHER_num(cipher_stack); i++)
 			{
-			SSL_CIPHER *c = sk_SSL_CIPHER_value(cipher_stack, i);
+			const SSL_CIPHER *c = sk_SSL_CIPHER_value(cipher_stack, i);
 
 			alg_k = c->algorithm_mkey;
 			alg_a = c->algorithm_auth;