internal/poll: add GetsockoptInt

internal/poll: add GetsockoptInt

This new function wraps the getsockopt network call with an integer
argument, similar to SetsockoptInt.

This will be used in MPTCP in the following commit.

This work has been co-developed by Gregory Detal
<gregory.detal@tessares.net>.

Updates #59166

Change-Id: I8f6aa00ea2535683d9bbf436993c23e9c6ca2af3
Reviewed-on: https://go-review.googlesource.com/c/go/+/471139
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>