Rename SSH2_KEX_MAX_HASH_LEN to be more general.

I'm about to want to use it for purposes other than KEX, so it's now
just called MAX_HASH_LEN and is supposed to be an upper bound on any
hash function we implement at all. Of course this makes no difference
to its value, because the largest hash we have is SHA-512 which
already fit inside that limit.

ssh.h

@@ -859,8 +859,8 @@ struct ssh2_userkey {
     char *comment;		       /* the key comment */
 };
 
-/* The maximum length of any hash algorithm used in kex. (bytes) */
-#define SSH2_KEX_MAX_HASH_LEN (64) /* SHA-512 */
+/* The maximum length of any hash algorithm. (bytes) */
+#define MAX_HASH_LEN (64)              /* longest is SHA-512 */
 
 extern const struct ssh1_cipheralg ssh1_3des;
 extern const struct ssh1_cipheralg ssh1_des;

ssh2transport.h

@@ -130,7 +130,7 @@ struct ssh2_transport_state {
     const struct ssh_kex *kex_alg;
     const ssh_keyalg *hostkey_alg;
     char *hostkey_str; /* string representation, for easy checking in rekeys */
-    unsigned char session_id[SSH2_KEX_MAX_HASH_LEN];
+    unsigned char session_id[MAX_HASH_LEN];
     int session_id_len;
     int dh_min_size, dh_max_size;
     bool dh_got_size_bounds;
@@ -177,7 +177,7 @@ struct ssh2_transport_state {
     ssh_key *hkey;                     /* actual host key */
     struct RSAKey *rsa_kex_key;             /* for RSA kex */
     ecdh_key *ecdh_key;                     /* for ECDH kex */
-    unsigned char exchange_hash[SSH2_KEX_MAX_HASH_LEN];
+    unsigned char exchange_hash[MAX_HASH_LEN];
     bool can_gssapi_keyex;
     bool need_gss_transient_hostkey;
     bool warned_about_no_gss_transient_hostkey;

sshrsa.c

@@ -727,9 +727,9 @@ static void oaep_mask(const struct ssh_hashalg *h, void *seed, int seedlen,
     while (datalen > 0) {
         int i, max = (datalen > h->hlen ? h->hlen : datalen);
         ssh_hash *s;
-        unsigned char hash[SSH2_KEX_MAX_HASH_LEN];
+        unsigned char hash[MAX_HASH_LEN];
 
-	assert(h->hlen <= SSH2_KEX_MAX_HASH_LEN);
+	assert(h->hlen <= MAX_HASH_LEN);
         s = ssh_hash_new(h);
         put_data(s, seed, seedlen);
         put_uint32(s, count);