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.

tests/test_security_curve.cpp

@@ -102,11 +102,10 @@ static void zap_handler (void *handler)
 
 int main (void)
 {
-#ifndef HAVE_LIBSODIUM
-    printf ("libsodium not installed, skipping CURVE test\n");
+#ifndef ZMQ_HAVE_CURVE
+    printf ("CURVE encryption not installed, skipping test\n");
     return 0;
 #endif
-
     //  Generate new keypairs for this test
     int rc = zmq_curve_keypair (client_public, client_secret);
     assert (rc == 0);