tcp_server_example: perform mknod(2) instead of open(2)

To establish TCP server we have to create a server file
So the example used to do open(O_CREAT) to create a file
But the system call above also performs `open` operation
This behavior causes blocking of open(2) until a new client connects to
Therefore now use mknod(path, S_IFREG | perm, 0) to resolve this issue
gitignore: ignore example programs