zmq_setsockopt should allow to set ZMQ_MULTICAST_HOPS to 0

[NoAnyLove]

Issue description

According to the following references, the valid TTL, which is referred as multicast hops in zmq, is from 0 to 255.

• https://tldp.org/HOWTO/Multicast-HOWTO-2.html#ss2.3
• https://tldp.org/HOWTO/Multicast-HOWTO-6.html#ss6.2

It is useful to set it to 0 when user wants to multicast data to consumers on the same host.

However, several checks in zmq code prevent user to set ZMQ_MULTICAST_HOPS to , e.g.,

• https://github.com/zeromq/libzmq/blob/v4.3.4/src/options.cpp#L437
• https://github.com/zeromq/libzmq/blob/v4.3.4/src/udp_engine.cpp#L143

Besides, it does not have upper limit check either (<= 255).

Minimal test code / Steps to reproduce the issue

#include <stdio.h>
#include <zmq.h>

int main() {
  void* ctx = zmq_ctx_new();
  void* socket = zmq_socket(ctx, ZMQ_RADIO);
  int value = 0;
  int rc = zmq_setsockopt(socket, ZMQ_MULTICAST_HOPS, &value, sizeof(value));
  printf("rc=%d\n", rc);
}

What's the actual result? (include assertion message & call stack if applicable)

It outputs,

rc=-1

What's the expected result?

I'm expecting it works and rc returns 0.