internal/poll, internal/syscall/unix, net: move and export fcntl syscall wrapper

internal/poll, internal/syscall/unix, net: move and export fcntl syscall wrapper

This will allow to use the fcntl syscall in packages other than
internal/poll.

For #60181

Change-Id: I76703766a655f2343c61dad95faf81aad58e007f
Reviewed-on: https://go-review.googlesource.com/c/go/+/494916
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>