systemd uses SIGRTMIN+n signals, but containerd didn't support the signals
since Go's sys/unix doesn't support them.
This change introduces SIGRTMIN+n handling by utilizing moby/sys/signal.
Fixes#5402.
https://www.freedesktop.org/software/systemd/man/systemd.html#Signals
Signed-off-by: Kazuyoshi Kato <katokazu@amazon.com>
