Flesh out the mmutex binary wrapper and test for existence of lockfile

mmutex/lib/flock.go

+// Copyright 2017 The LUCI Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+package lib
+
+import (
+        "os"
+
+        "github.com/luci/luci-go/common/errors"
+)
+
+// TODO(charliea): Add timeout parameter to this function.
+func AcquireExclusiveLock(path string) error {
+        _, err := os.OpenFile(path, os.O_APPEND, 0666)
+        if os.IsNotExist(err) {
+                return errors.Reason("lock file does not exist", path).Err()
+        }
+
+        panic("implement lock acquisition")
+}