[#LIBZMQ-446] Silence error on setting PGM_TOS due to some platforms raising an…

[#LIBZMQ-446] Silence error on setting PGM_TOS due to some platforms raising an error at runtime. Noted are RHEL 4.

src/pgm_socket.cpp

@@ -336,10 +336,11 @@ int zmq::pgm_socket_t::init (bool udp_encapsulation_, const char *network_)
 		    goto err_abort;
 
 		//  Expedited Forwarding PHB for network elements, no ECN.
+		//  Ignore return value due to varied runtime support.
 		const int dscp = 0x2e << 2;
-		if (AF_INET6 != sa_family && !pgm_setsockopt (sock,
-		      IPPROTO_PGM, PGM_TOS, &dscp, sizeof (dscp)))
-		    goto err_abort;
+		if (AF_INET6 != sa_family)
+		    pgm_setsockopt (sock, IPPROTO_PGM, PGM_TOS,
+		       &dscp, sizeof (dscp));
 
 		const int nonblocking = 1;
 		if (!pgm_setsockopt (sock, IPPROTO_PGM, PGM_NOBLOCK,