func (fds *FDSet) Set(fd uintptr) {
	fds.X__fds_bits[fd/NFDBITS] |= (1 << (fd % NFDBITS))
}
Set adds the fd to the set
package goselect
import "syscall"
const FD_SETSIZE = syscall.FD_SETSIZE
type FDSet syscall.FdSet