Add pidfile support

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

Fixes #320

vendor/golang.org/x/sys/windows/eventlog.go

@@ -0,0 +1,20 @@
+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build windows
+
+package windows
+
+const (
+	EVENTLOG_SUCCESS          = 0
+	EVENTLOG_ERROR_TYPE       = 1
+	EVENTLOG_WARNING_TYPE     = 2
+	EVENTLOG_INFORMATION_TYPE = 4
+	EVENTLOG_AUDIT_SUCCESS    = 8
+	EVENTLOG_AUDIT_FAILURE    = 16
+)
+
+//sys	RegisterEventSource(uncServerName *uint16, sourceName *uint16) (handle Handle, err error) [failretval==0] = advapi32.RegisterEventSourceW
+//sys	DeregisterEventSource(handle Handle) (err error) = advapi32.DeregisterEventSource
+//sys	ReportEvent(log Handle, etype uint16, category uint16, eventId uint32, usrSId uintptr, numStrings uint16, dataSize uint32, strings **uint16, rawData *byte) (err error) = advapi32.ReportEventW