Fix signal (SIGINT) handling when running kubectl with terminal attached

[r10r]

Currently it's not possible to interrupt the following command using Control-C

kubectl run -i --tty ping --image=busybox --restart=Never --rm --privileged=true -- ping github.com

Check if lxcri-init must set the signal mask before executing the container process.
The container process should inherit the signal mask from lxcri-init.
See libc manual - 24.7.3 Process Signal Mask