fslock-guard — A guard object to ensure we have an exclusive lock to a file

This crate is a thin wrapper around fslock, which uses flock(2) or LockFileEx to acquire an advisory lock on the filesystem.

We add two features that fslock does not (currently) have:

• We have a [LockFileGuard] type, which can be used to ensure that a lock is actually held until the guard is dropped.
• We perform a post-lock check to make sure that our lockfile has not been removed and re-created on disk by someone else. This check makes it safe to remove lockfiles.