pkg/signal: move signal.DumpStacks() to a separate package

It is not directly related to signal-handling, so can well live
in its own package.

Also added a variant that doesn't take a directory to write files
to, for easier consumption / better match to how it's used.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

daemon/debugtrap_unix.go

@@ -6,7 +6,7 @@ import (
 	"os"
 	"os/signal"
 
-	stackdump "github.com/docker/docker/pkg/signal"
+	"github.com/docker/docker/pkg/stack"
 	"github.com/sirupsen/logrus"
 	"golang.org/x/sys/unix"
 )
@@ -16,7 +16,7 @@ func (daemon *Daemon) setupDumpStackTrap(root string) {
 	signal.Notify(c, unix.SIGUSR1)
 	go func() {
 		for range c {
-			path, err := stackdump.DumpStacks(root)
+			path, err := stack.DumpToFile(root)
 			if err != nil {
 				logrus.WithError(err).Error("failed to write goroutines dump")
 			} else {