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_unix.go

@@ -0,0 +1,21 @@
+// +build !windows
+
+package signal
+
+import (
+	"syscall"
+)
+
+// Signals used in cli/command (no windows equivalent, use
+// invalid signals so they don't get handled)
+
+const (
+	// SIGCHLD is a signal sent to a process when a child process terminates, is interrupted, or resumes after being interrupted.
+	SIGCHLD = syscall.SIGCHLD
+	// SIGWINCH is a signal sent to a process when its controlling terminal changes its size
+	SIGWINCH = syscall.SIGWINCH
+	// SIGPIPE is a signal sent to a process when a pipe is written to before the other end is open for reading
+	SIGPIPE = syscall.SIGPIPE
+	// DefaultStopSignal is the syscall signal used to stop a container in unix systems.
+	DefaultStopSignal = "SIGTERM"
+)