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_windows.go

@@ -0,0 +1,36 @@
+package signal
+
+import (
+	"syscall"
+
+	"golang.org/x/sys/windows"
+)
+
+// Signals used in cli/command (no windows equivalent, use
+// invalid signals so they don't get handled)
+const (
+	SIGCHLD  = syscall.Signal(0xff)
+	SIGWINCH = syscall.Signal(0xff)
+	SIGPIPE  = syscall.Signal(0xff)
+	// DefaultStopSignal is the syscall signal used to stop a container in windows systems.
+	DefaultStopSignal = "15"
+)
+
+// SignalMap is a map of "supported" signals. As per the comment in GOLang's
+// ztypes_windows.go: "More invented values for signals". Windows doesn't
+// really support signals in any way, shape or form that Unix does.
+var SignalMap = map[string]syscall.Signal{
+	"HUP":  syscall.Signal(windows.SIGHUP),
+	"INT":  syscall.Signal(windows.SIGINT),
+	"QUIT": syscall.Signal(windows.SIGQUIT),
+	"ILL":  syscall.Signal(windows.SIGILL),
+	"TRAP": syscall.Signal(windows.SIGTRAP),
+	"ABRT": syscall.Signal(windows.SIGABRT),
+	"BUS":  syscall.Signal(windows.SIGBUS),
+	"FPE":  syscall.Signal(windows.SIGFPE),
+	"KILL": syscall.Signal(windows.SIGKILL),
+	"SEGV": syscall.Signal(windows.SIGSEGV),
+	"PIPE": syscall.Signal(windows.SIGPIPE),
+	"ALRM": syscall.Signal(windows.SIGALRM),
+	"TERM": syscall.Signal(windows.SIGTERM),
+}