connhelper: export functions for other projects

Exposed functions are planned to be used by `buildctl`:
https://github.com/moby/buildkit/issues/769

Signed-off-by: Akihiro Suda <suda.akihiro@lab.ntt.co.jp>

cli/connhelper/commandconn/commandconn_linux.go

@@ -0,0 +1,12 @@
+package commandconn
+
+import (
+	"os/exec"
+	"syscall"
+)
+
+func setPdeathsig(cmd *exec.Cmd) {
+	cmd.SysProcAttr = &syscall.SysProcAttr{
+		Pdeathsig: syscall.SIGKILL,
+	}
+}