Package flock provides best-effort advisory file locking using flock(2).
This is used for cross-replica coordination (e.g. preventing duplicate
ACME requests). Note that flock(2) does NOT work reliably on NFS volumes:
on NFSv3 it depends on the NLM daemon, on NFSv4 Linux emulates it via
fcntl locks with different semantics. Callers must treat lock failures
as non-fatal and proceed without the lock.
Lock acquires an exclusive advisory lock on the given file path.
It creates the lock file if it does not exist. The lock attempt
respects context cancellation by using non-blocking flock with polling.
The caller must call Unlock with the returned *os.File when done.
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