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 (and file access mode flag like O_RDONLY, but that will be ignored).

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

[RalfJung]

Here's the first round of comments. Sorry for the long wait!

@rustbot author

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[tiif]

On the latest commit b22019d, I made all the tests in this PR to use isolation mode.

[tiif]

Just a heads up, this PR will stall for a while due to exam prep, but I will be back on it after rust week.

[tiif]

All feedback has been addressed :)

@rustbot ready

[tiif]

Is rustbot taking a break?

@rustbot label -S-waiting-on-author +S-waiting-on-review

[tiif]

Trying one last time...

@rustbot label -S-waiting-on-author

[RalfJung]

Thanks. :)
@rustbot author

[tiif]

@rustbot ready

[RalfJung]

We're almost there, just a few nits. :)
@rustbot author

[RalfJung]

You know you can land multiple GH suggestions as a single commit? :) Just go to the diff view, there's a button for that.

[tiif]

Ohh I didn't know that! Clicking one by one is indeed a bit annoying because the webpage will refresh for every commit.

Ah I saw the button now :)

[tiif]

@rustbot ready

[RalfJung]

This looks great, thanks! Please squash the commits using ./miri squash. Then write @rustbot ready after you force-pushed the squashed PR.

@rustbot author

[tiif]

Done, thanks for the review :)

@rustbot ready