NFS-safe file locking with timeouts for POSIX and Windows.

The flufl.lock library provides an NFS-safe file-based locking
algorithm influenced by the GNU/Linux open(2) manpage, under the
description of the O_EXCL option.

The assumption made here is that there will be no outside
interference, e.g. no agent external to this code will ever link() to
the specific lock files used.

Lock objects support lock-breaking so that you can’t wedge a process
forever. This is especially helpful in a web environment, but may not
be appropriate for all applications.