tests: Add new "procutils" that exposes PDEATHSIG

To fix compilation on MacOS.

I think actually we want to use this pervasively in our tests
on Linux; it doesn't really matter when run inside a transient
container, but `PDEATHSIG` is useful for persistent containers (e.g.)
toolbox and when running outside of a pid namespace, e.g. on a host
system shell directly or in systemd.

Signed-off-by: Colin Walters <walters@verbum.org>

integration/proxy_test.go

@@ -186,9 +186,7 @@ func newProxy() (*proxy, error) {
 	// Note ExtraFiles starts at 3
 	proc := exec.Command("skopeo", "experimental-image-proxy", "--sockfd", "3")
 	proc.Stderr = os.Stderr
-	proc.SysProcAttr = &syscall.SysProcAttr{
-		Pdeathsig: syscall.SIGTERM,
-	}
+	cmdLifecycleToParentIfPossible(proc)
 	proc.ExtraFiles = append(proc.ExtraFiles, theirfd)
 
 	if err = proc.Start(); err != nil {