backend: move backend implementation helpers to util package

This removes code that is only used within a backend implementation from
the backend package. The latter now only contains code that also has
external users.

internal/backend/util/foreground_windows.go

@@ -0,0 +1,22 @@
+package util
+
+import (
+	"os/exec"
+	"syscall"
+
+	"github.com/restic/restic/internal/errors"
+	"golang.org/x/sys/windows"
+)
+
+func startForeground(cmd *exec.Cmd) (bg func() error, err error) {
+	// just start the process and hope for the best
+	cmd.SysProcAttr = &syscall.SysProcAttr{}
+	cmd.SysProcAttr.CreationFlags = windows.CREATE_NEW_PROCESS_GROUP
+	err = cmd.Start()
+	if err != nil {
+		return nil, errors.Wrap(err, "cmd.Start")
+	}
+
+	bg = func() error { return nil }
+	return bg, nil
+}