Add resume feature

Added an interface and machinery for resuming failed uploads.
Implemented this interface in the local backend.
Later on it can be implemented by any supporting backend.

Fixes #87

fs/operations/interrupt_test.go

@@ -0,0 +1,23 @@
+//go:build !windows
+// +build !windows
+
+package operations
+
+import (
+	"os"
+	"os/exec"
+	"syscall"
+)
+
+func sendInterrupt() error {
+	p, err := os.FindProcess(syscall.Getpid())
+	if err != nil {
+		return err
+	}
+	err = p.Signal(os.Interrupt)
+	return err
+}
+
+func setupCmd(cmd *exec.Cmd) {
+	// Only needed for windows
+}