Clean up after using randombytes from libsodium

When Curve authentication is used, libsodium opens a file
descriptor to /dev/urandom to generate random bytes. When
the ZMQ context terminates, it should ensure that file gets
closed.

src/ctx.cpp

@@ -35,6 +35,10 @@
 #include "err.hpp"
 #include "msg.hpp"
 
+#ifdef HAVE_LIBSODIUM
+#include <sodium.h>
+#endif
+
 #define ZMQ_CTX_TAG_VALUE_GOOD 0xabadcafe
 #define ZMQ_CTX_TAG_VALUE_BAD  0xdeadbeef
 
@@ -89,6 +93,12 @@ zmq::ctx_t::~ctx_t ()
     //  corresponding io_thread/socket objects.
     free (slots);
 
+    //  If we've done any Curve encryption, we may have a file handle
+    //  to /dev/urandom open that needs to be cleaned up.
+#ifdef HAVE_LIBSODIUM
+    randombytes_close();
+#endif
+
     //  Remove the tag, so that the object is considered dead.
     tag = ZMQ_CTX_TAG_VALUE_BAD;
 }