Add GetParentDeathSignal() to pkg/system

Docker-DCO-1.1-Signed-off-by: Bernerd Schaefer <bj.schaefer@gmail.com> (github: bernerdschaefer)
nsinit.Init() restores parent death signal before exec

Docker-DCO-1.1-Signed-off-by: Bernerd Schaefer <bj.schaefer@gmail.com> (github: bernerdschaefer)
nsinit.DefaultCreateCommand sets Pdeathsig to SIGKILL

Docker-DCO-1.1-Signed-off-by: Bernerd Schaefer <bj.schaefer@gmail.com> (github: bernerdschaefer)
Merge pull request #5792 from bernerdschaefer/nsinit-supports-pdeathsig

Add PDEATHSIG support to nsinit library