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_windows.go

@@ -0,0 +1,25 @@
+package pidfile
+
+import (
+	"golang.org/x/sys/windows"
+)
+
+const (
+	processQueryLimitedInformation = 0x1000
+
+	stillActive = 259
+)
+
+func processExists(pid int) bool {
+	h, err := windows.OpenProcess(processQueryLimitedInformation, false, uint32(pid))
+	if err != nil {
+		return false
+	}
+	var c uint32
+	err = windows.GetExitCodeProcess(h, &c)
+	windows.Close(h)
+	if err != nil {
+		return c == stillActive
+	}
+	return true
+}