Implement ZMQ_TCP_ACCEPT_FILTER setsockopt() for listening TCP sockets.

Assign arbitrary number of filters that will be applied for each new TCP transport
connection on a listening socket.
If no filters applied, then TCP transport allows connections from any ip.
If at least one filter is applied then new connection source ip should be matched.
To clear all filters call zmq_setsockopt(socket, ZMQ_TCP_ACCEPT_FILTER, NULL, 0).
Filter is a null-terminated string with ipv6 or ipv4 CIDR.

For example:
localhost
127.0.0.1
mail.ru/24
::1
::1/128
3ffe:1::
3ffe:1::/56

Returns -1 if the filter couldn't be assigned(format error or ipv6 filter with ZMQ_IPV4ONLY set)

P.S.
The only thing that worries me is that I had to re-enable 'default assign by reference constructor/operator'
for 'tcp_address_t' (and for my inherited class tcp_address_mask_t) to store it in std::vector in 'options_t'...

src/options.hpp

@@ -24,9 +24,11 @@
 #define __ZMQ_OPTIONS_HPP_INCLUDED__
 
 #include <string>
+#include <vector>
 
 #include "stddef.h"
 #include "stdint.hpp"
+#include "tcp_address.hpp"
 
 namespace zmq
 {
@@ -118,6 +120,10 @@ namespace zmq
         int tcp_keepalive_idle;
         int tcp_keepalive_intvl;
 
+        // TCP accept() filters
+        typedef std::vector <tcp_address_mask_t> tcp_accept_filters_t;
+        tcp_accept_filters_t tcp_accept_filters;
+
         //  ID of the socket.
         int socket_id;
     };