internal/poll: fcntl() should loop on EINTR

[networkimprov]

The internal/poll.fcntl sites need an EINTR loop. The docs list EINTR for some features, and darwin F_FULLFSYNC hits the disk.
https://man7.org/linux/man-pages/man2/fcntl.2.html

https://github.com/golang/go/blob/master/src/internal/poll/fcntl_libc.go
https://github.com/golang/go/blob/master/src/internal/poll/fcntl_syscall.go
https://github.com/golang/go/blob/master/src/internal/poll/fd_fsync_darwin.go
possibly others...

It might be worth a comment on poll.CloseFunc, to note that errors don't mean failure, so retry is incorrect:
https://man7.org/linux/man-pages/man2/close.2.html

Also, seems like a backport fix...

@gopherbot add NeedsFix
cc @ianlancetaylor

[ianlancetaylor]

Note that we never call fcntl with the operations that the GNU/Linux man page says can return EINTR.

I guess we do need a loop on Darwin, though.

[ianlancetaylor]

We aren't going to backport any changes in this area, any EINTR errors are not a regression.

[networkimprov]

I'd worry that some other fcntl() op might return EINTR -- if not now, later.

Could we tag for 1.16?

[gopherbot]

Change https://golang.org/cl/258542 mentions this issue: internal/poll: use ignoringEINTR in Darwin Fsync