quic: avoid depending on golang.org/x/sys/unix

To test our HTTP/3 implementation against tests in std, we need to
vendor the http3 package. However, http3 relies on quic, which relies on
x/sys/unix.

Vendoring x/sys/unix in std is not allowed (golang/go#32102). Therefore,
replace x/sys/unix usage with syscall to avoid the dependency.

For golang/go#70914

Change-Id: Iad8efff92c9f4135a0d8b89f8544fa206a6a6964
Reviewed-on: https://go-review.googlesource.com/c/net/+/763160
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: Nicholas Husin <husin@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>