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_sysv.go

@@ -0,0 +1,28 @@
+//go:build aix || solaris
+// +build aix solaris
+
+package util
+
+import (
+	"os/exec"
+	"syscall"
+
+	"github.com/restic/restic/internal/errors"
+)
+
+func startForeground(cmd *exec.Cmd) (bg func() error, err error) {
+	// run the command in it's own process group so that SIGINT
+	// is not sent to it.
+	cmd.SysProcAttr = &syscall.SysProcAttr{
+		Setpgid: true,
+	}
+
+	// start the process
+	err = cmd.Start()
+	if err != nil {
+		return nil, errors.Wrap(err, "cmd.Start")
+	}
+
+	bg = func() error { return nil }
+	return bg, nil
+}