os: don't try to signal PID -1 on Unix

os: don't try to signal PID -1 on Unix

This restores behavior that we lost in CL 588675.

Fixes #68496

Change-Id: I1740986bed647835986d54109071b7a6b37413d5
Reviewed-on: https://go-review.googlesource.com/c/go/+/599015
Reviewed-by: Cherry Mui <cherryyz@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Commit-Queue: Ian Lance Taylor <iant@google.com>