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/procutils_linux.go

@@ -0,0 +1,14 @@
+package main
+
+import (
+	"os/exec"
+	"syscall"
+)
+
+// cmdLifecyleToParentIfPossible is a thin wrapper around prctl(PR_SET_PDEATHSIG)
+// on Linux.
+func cmdLifecycleToParentIfPossible(c *exec.Cmd) {
+	c.SysProcAttr = &syscall.SysProcAttr{
+		Pdeathsig: syscall.SIGTERM,
+	}
+}