File#fsync, File#flock_exclusive, File#flock_shared, and File#flock_unlock belong in IO::FileDescriptor

[BrucePerens]

Unix-style file operations, like File#fsync, File#flock_exclusive, File#flock_shared, and File#flock_unlock belong in IO::FileDescriptor. On Windows these are completely different APIs, which perhaps is why they are presently in File. However, this is a significant handicap on Unix-like systems and makes the user want to import the API from LibC, which would be ugly and even less portable.

An example of how to code these APIs portably to Windows is at https://github.com/WoLpH/portalocker

Why is the user using IO::FileDescriptor? They are doing operations on STDIN, STDOUT, or STDERR (which can be legitimate files), or on a file descriptor inherited from the parent process, or passed in from a foreign library.

[asterite]

I think this was just a short sight when we added these methods. PRs are welcome to move them to IO::FileDescriptor. It should be pretty straight-forward.