Patched INITIATE message following Daniel Boucher

I was looking at the file curve_codec.c and I found a little issue in
the implementation of the INITIATE message when computing Box [C',S](C->S')
At some point there was a fix made (#14) referring to
https://codesinchaos.wordpress.com/2012/09/09/curvecp-1 but I think a
bug got introduced in the code

NOTE: The following problem doesn't apply in the ZMQ's implementation,
the code in curve_client.cpp and curve_server.cpp seems correct

The actual code at line 627 for s_produce_initiate() is

    s_encrypt (self, vouch_crypt,
               vouch_plain, 64,
               "VOUCH---",
               NULL, NULL);

So it's using the precomputed key (C'->S') instead of (C->S'), if I'm
not mistaking to the correct code should be:

    s_encrypt (self, vouch_crypt,
               vouch_plain, 64,
               "VOUCH---",
               self->peer_transkey,
               zcert_secret_key (self->permacert));

Same thing on the server side for processing the initiate message at
line 704 (process_initiate()):

        int rc = s_decrypt (self,
            vouch,
            plain, 64,
            "VOUCH---",
            NULL, NULL);

The correct behavior would be:

        int rc = s_decrypt (self,
            vouch,
            plain, 64,
            "VOUCH---",
            self->peer_permakey,
            zcert_secret_key(self->transcert));