Problem: use of libsodium vs. tweetnacl is confused

It's unclear which we need and in the source code, conditional code
treats tweetnacl as a subclass of libsodium, which is inaccurate.

Solution: redesign the configure/cmake API for this:

* tweetnacl is present by default and cannot be enabled
* libsodium can be enabled using --with-libsodium, which replaces
  the built-in tweetnacl
* CURVE encryption can be disabled entirely using --enable-curve=no

The macros we define in platform.hpp are:

    ZMQ_HAVE_CURVE    1        //  When CURVE is enabled
    HAVE_LIBSODIUM    1        //  When we are using libsodium
    HAVE_TWEETNACL    1        //  When we're using tweetnacl (default)

As of this patch, the default build of libzmq always has CURVE
security, and always uses tweetnacl.

src/stream_engine.cpp

@@ -682,7 +682,7 @@ bool zmq::stream_engine_t::handshake ()
                     plain_client_t (options);
             alloc_assert (mechanism);
         }
-#ifdef HAVE_LIBSODIUM
+#ifdef ZMQ_HAVE_CURVE
         else
         if (options.mechanism == ZMQ_CURVE
         &&  memcmp (greeting_recv + 12, "CURVE\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0", 20) == 0) {