unix: add GetPeerUcred and UcredGet for solaris

Change-Id: I74ba119fb729ef46899de04c686115f960975bb3
Reviewed-on: https://go-review.googlesource.com/c/sys/+/639755
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Matt Layher <mdlayher@gmail.com>