Support SIGRTMIN+n signals

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>

vendor/github.com/moby/sys/signal/signal_unsupported.go

@@ -0,0 +1,10 @@
+// +build !linux,!darwin,!freebsd,!windows
+
+package signal
+
+import (
+	"syscall"
+)
+
+// SignalMap is an empty map of signals for unsupported platform.
+var SignalMap = map[string]syscall.Signal{}