* The INITIATE command vouch box is Box[C',S](C->S') instead of Box[C'](C->S),
  as recommended by https://codesinchaos.wordpress.com/2012/09/09/curvecp-1/,
  to reduce the risk of client impersonation.

* Mirrors the change in libcurve and CurveZMQ specifications.

This reverts commit bfd472f9.

* Command names changed from null terminated to length-specified
* Command frames use the correct flag (bit 2)
* test_stream acts as test case for command frames
* Some code cleanups

RFC23, RFC24, RFC26 now use variable-length command names that
end in null octet (valid C strings) instead of fixed-length
space padded strings.

- ZMQ_CURVE_PUBLICKEY for clients and servers
- ZMQ_CURVE_SECRETKEY for clients
- ZMQ_CURVE_SERVERKEY for clients
- ZMQ_CURVE_SERVER for servers
- added tools/curve_keygen.c as example
- updated man pages