support set IP_DONTFRAG #318

 //setsockopt(IP_DONTFRAG)issupportedonmacOSfromBigSur
 #elifBUILDFLAG(IS_MAC)
 if(!base::mac::IsAtLeastOS11()){
 returnERR_NOT_IMPLEMENTED;
 }
 intval=1;
 if(addr_family_==AF_INET6){
 intrv=
 setsockopt(socket_,IPPROTO_IPV6,IPV6_DONTFRAG,&val,sizeof(val));
 //IP_DONTFRAGisnotsupportedonv4mappedaddresses.
 returnrv==0?OK:MapSystemError(errno);
 }
 intrv=setsockopt(socket_,IPPROTO_IP,IP_DONTFRAG,&val,sizeof(val));
 returnrv==0?OK:MapSystemError(errno);
 ```