Add 'next_message' methods to cipher and MAC vtables.

This provides a convenient hook to be called between SSH messages, for
the crypto components to do any per-message processing like
incrementing a sequence number.

utils/CMakeLists.txt

@@ -42,7 +42,9 @@ add_sources_from_current_dir(utils
   memory.c
   memxor.c
   null_lp.c
+  nullcipher.c
   nullkey.c
+  nullmac.c
   nullseat.c
   nullstrcmp.c
   out_of_memory.c

utils/nullcipher.c

@@ -0,0 +1,11 @@
+/*
+ * Implementation of shared trivial routines that ssh_cipher
+ * implementations might use.
+ */
+
+#include "ssh.h"
+
+void nullcipher_next_message(ssh_cipher *cipher)
+{
+    /* Most ciphers don't do anything at all with this */
+}

utils/nullmac.c

@@ -0,0 +1,11 @@
+/*
+ * Implementation of shared trivial routines that ssh2_mac
+ * implementations might use.
+ */
+
+#include "ssh.h"
+
+void nullmac_next_message(ssh2_mac *m)
+{
+    /* Most MACs don't do anything at all with this */
+}