Fix rvalue for socket::send() on EHOSTUNREACH

During introduction of EHOSTUNREACH, missing mapping between
EHOSTUNREACH errno and false/0 return code for socket's send()
calls was missing, throwing an exception.

To be consistent with the rest of wrappers (e.g. DONTWAIT), fix it
by handling this errno as a regular EAGAIN, and let the caller use
errno/zmq_errno() to branch on their code.

zmq.hpp

@@ -631,6 +631,8 @@ namespace zmq
                 return (size_t) nbytes;
             if (zmq_errno () == EAGAIN)
                 return 0;
+            if (zmq_errno () == EHOSTUNREACH)
+                return 0;
             throw error_t ();
         }
 
@@ -641,6 +643,8 @@ namespace zmq
                 return true;
             if (zmq_errno () == EAGAIN)
                 return false;
+            if (zmq_errno () == EHOSTUNREACH)
+                return false;
             throw error_t ();
         }