support set IP_DONTFRAG

[gcxfd]

see

https://chromium.googlesource.com/chromium/src/net/+/refs/heads/main/socket/udp_socket_posix.cc

 //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);
 ```

[Thomasdezeeuw]

@gcxfd do you mean you would like a function that sets this flag? If so, such a change would be welcome.

[jcdutton]

I have put a patch to add this in:
#487

Only tested on Linux.

[Thomasdezeeuw]

I have put a patch to add this in: #487

Only tested on Linux.

Can you please send a pull requests?