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>

go.mod

@@ -37,6 +37,7 @@ require (
 	github.com/klauspost/compress v1.11.13
 	github.com/moby/locker v1.0.1
 	github.com/moby/sys/mountinfo v0.4.1
+	github.com/moby/sys/signal v0.5.1-0.20210723232958-8a51b5cc8879
 	github.com/moby/sys/symlink v0.1.0
 	github.com/opencontainers/go-digest v1.0.0
 	github.com/opencontainers/image-spec v1.0.1
@@ -55,7 +56,7 @@ require (
 	go.etcd.io/bbolt v1.3.5
 	golang.org/x/net v0.0.0-20210226172049-e18ecbb05110
 	golang.org/x/sync v0.0.0-20201207232520-09787c993a3a
-	golang.org/x/sys v0.0.0-20210426230700-d19ff857e887
+	golang.org/x/sys v0.0.0-20210630005230-0f9fa26af87c
 	google.golang.org/grpc v1.38.0
 	google.golang.org/protobuf v1.26.0
 	gotest.tools/v3 v3.0.3