Op lock_fh has different sematics on windows and *nix

[retupmoca]

lock_fh uses fcntl on *nix systems, which is reentrant in the sense that if a process tries to lock a file it has already locked, the lock is a no-op (and a call to 'unlock' (or closing a file) will remove all locks the current process holds on a file)

On Windows sytems, lock_fh uses LockFileEx, which will block if the same process attempts to lock a file twice.