Problem: CURVE mechanism is unusable with chroot()

libsodium calls abort() when /dev/urandom can't be found
even if one creates ZeroMQ context before calling chroot()[1].

This happens because crypto gets initialized on handshake,
and at that moment the process is already chroot'ed.

Solution: initialize cryptographic libraries in ctx

randombytes_close() is already there in the destructor.

[1] https://download.libsodium.org/doc/usage/index.html

src/ctx.hpp

@@ -233,6 +233,8 @@ namespace zmq
         int vmci_family;
         mutex_t vmci_sync;
 #endif
+
+        mutex_t crypto_sync;
     };
 
 }