1) Don't mswait(1) on a successful lock, or when timeout is zero.
2) Flush read buffer on Get and ReLoad()
   - If there's a single record in the file, close and re-open it.
   - If a lock is held on a different record already, read that record.
   - Otherwise, read the first record a lock with timeout zero succeeds.
3) Flush write buffer on Put().

This should now actually be safe as long as locks work.
Using file_mutex() on records is the only potential change still needed.
(Because POSIX fcntl() locks are insane)