Add pidfile support

Copy a simple implementation from the Moby project, since importing
their package would pull in too many dependencies.

Fixes #320

internal/pidfile/pidfile_test.go

@@ -0,0 +1,38 @@
+package pidfile
+
+import (
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestNewAndRemove(t *testing.T) {
+	dir, err := ioutil.TempDir(os.TempDir(), "test-pidfile")
+	if err != nil {
+		t.Fatal("Could not create test directory")
+	}
+
+	path := filepath.Join(dir, "testfile")
+	file, err := New(path)
+	if err != nil {
+		t.Fatal("Could not create test file", err)
+	}
+
+	_, err = New(path)
+	if err == nil {
+		t.Fatal("Test file creation not blocked")
+	}
+
+	if err := file.Remove(); err != nil {
+		t.Fatal("Could not delete created test file")
+	}
+}
+
+func TestRemoveInvalidPath(t *testing.T) {
+	file := PIDFile{path: filepath.Join("foo", "bar")}
+
+	if err := file.Remove(); err == nil {
+		t.Fatal("Non-existing file doesn't give an error on delete")
+	}
+}