Explicitly make GC roots client sockets blocking

On macOS / BSD, these sockets inherit the non-blocking flag of the
server soocket, which is not what we want.

https://github.com/dotnet/runtime/issues/25069
https://bugs.python.org/issue7995

https://hydra.nixos.org/build/161439304

src/libstore/gc.cc

@@ -535,6 +535,12 @@ void LocalStore::collectGarbage(const GCOptions & options, GCResults & results)
                         }
                     });
 
+                    /* On macOS, accepted sockets inherit the
+                       non-blocking flag from the server socket, so
+                       explicitly make it blocking. */
+                    if (fcntl(fdServer.get(), F_SETFL, fcntl(fdServer.get(), F_GETFL) & ~O_NONBLOCK) == -1)
+                        abort();
+
                     while (true) {
                         try {
                             auto path = readLine(fdClient.get());