Add the POSIX fcntl call, but only implementing F_SETFL and F_GETFL
for O_NONBLOCK, so users can control the blocking flag of streams
with only the integer file descriptor.

Necessary to portably control the blockingness of the console:

    int flags = fcntl(STDOUT_FILENO, F_GETFL);
    fcntl(STDOUT_FILENO, F_SETFL, flags | O_NONBLOCK);