Platform-independent file locking module

The lockfile module exports a FileLock class which provides a simple API for locking files. Unlike the Windows msvcrt.locking function, the Unix fcntl.flock, fcntl.lockf and the deprecated posixfile module, the API is identical across both Unix (including Linux and Mac) and Windows platforms. The lock mechanism relies on the atomic nature of the link (on Unix) and mkdir (on Windows) system calls.