Problem: tweetnacl sources are a mess

- they have no copyright / license statement
- they are in some randomish directory structure
- they are a mix of postable and non-portable files
- they do not conform to conditional compile environment

Overall, it makes it rather more work than needed, in build scripts.

Solution: clean up tweetnacl sauce.

- merged code into single tweetnacl.c and .h
- standard copyright header, DJB to AUTHORS
- moved into src/ along with all other source files
- all system and conditional compilation hidden in these files
- thus, they can be compiled and packaged in all cases
- ZMQ_USE_TWEETNACL is set when we're using built-in tweetnacl
- HAVE_LIBSODIUM is set when we're using external libsodium

src/zmq_utils.cpp

@@ -38,14 +38,13 @@
 #include <assert.h>
 
 #if !defined ZMQ_HAVE_WINDOWS
-#include <unistd.h>
+#   include <unistd.h>
 #else
-#include "windows.hpp"
+#   include "windows.hpp"
 #endif
 
-#if defined (HAVE_TWEETNACL)
-#   include "tweetnacl_base.h"
-#   include "randombytes.h"
+#if defined (ZMQ_USE_TWEETNACL)
+#   include "tweetnacl.h"
 #elif defined (HAVE_LIBSODIUM)
 #   include "sodium.h"
 #endif