Support F_GETFL and F_SETFL for fcntl

[tiif]

This PR supports F_SETFL and F_GETFL flags for fcntl. In this implementation, F_SETFL can only set O_NONBLOCK flag.

If setfl is called while a fd is blocking, that fd will not be affected by the new flag value and will continue blocking like normal until a read/write wakes it up (it acts like it never knew the setfl has happened). But any operation after setfl will see the new flag value.

The interaction between these fcntl operations and blocking fd is summarised here.

Fixes #4119

[tiif]

@rustbot ready

[tiif]

This is needed for

// We only support F_GETFL for socketpair and pipe.
let anonsocket_fd = fd.downcast::<AnonSocket>().ok_or_else(|| {
    err_unsup_format!("fcntl: only socketpair / pipe are supported for F_SETFL")
})?;

[tiif]

From the man page:

       F_SETFL (int)
              Set the file status flags to the value specified by arg.
              File access mode (O_RDONLY, O_WRONLY, O_RDWR) and file
              creation flags (i.e., O_CREAT, O_EXCL, O_NOCTTY, O_TRUNC)
              in arg are ignored.

There are quite a few file access mode and file creation flags, would prefer to open an issue for this and do it later.