Disable IPv6 support

The current IPv6 support is half-assed and breaks for too many people.
Revert back to IPv4 only for now.

src/ip.cpp

@@ -232,8 +232,8 @@ int zmq::resolve_ip_interface (sockaddr_storage* addr_, socklen_t *addr_len_,
     addrinfo req;
     memset (&req, 0, sizeof (req));
 
-    //  We don't care about family. IPv4 is as good as IPv6.
-    req.ai_family = AF_UNSPEC;
+    //  We only support IPv4 addresses for now.
+    req.ai_family = AF_INET;
 
     //  Arbitrary, not used in the output, but avoids duplicate results.
     req.ai_socktype = SOCK_STREAM;
@@ -280,15 +280,17 @@ int zmq::resolve_ip_hostname (sockaddr_storage *addr_, socklen_t *addr_len_,
     addrinfo req;
     memset (&req, 0, sizeof (req));
 
-    //  Don't specify the family. We want both IPv4 and IPv6.
-    req.ai_family = AF_UNSPEC;
+    //  We only support IPv4 addresses for now.
+    req.ai_family = AF_INET;
 
     //  Need to choose one to avoid duplicate results from getaddrinfo() - this
     //  doesn't really matter, since it's not included in the addr-output.
     req.ai_socktype = SOCK_STREAM;
     
-    //  Avoid named services due to unclear socktype, and don't pick IPv6
-    //  addresses if we don't have a local IPv6 address configured.
+    //  Avoid named services due to unclear socktype, and don't pick IPv4
+    //  addresses if we don't have a local IPv4 address configured.
+    //  If this is failing for you on a host with only IPv6 connectivity,
+    //  please contribute proper IPv6 support for all functions in this file.
     req.ai_flags = AI_NUMERICSERV | AI_ADDRCONFIG;
 
     //  Resolve host name. Some of the error info is lost in case of error,