Fix #2666: posix dirent constants are now parameterless methods

[LeeTibbert]

This PR introduces a small source incompatibility so it needs to be on the 0.5.0 train.

Posixlib dirent.scala now follows the convention across Scala Native posixlib
that Posix C constants are Scala parameterless (zero-arity) methods.

[ekrich]

I set the 0.5.0 milestone if that is ok - I am thinking you want this merge post 0.4.x. It was mentioned that there might be one more release before breaking changes like 32bit and experimental multi-threading.

[LeeTibbert]

re: I set the 0.5.0 milestone

Thank you. That makes my life easier.

I did not know that users could do that. Learn something every day.
I will have to check my other PRs intended for 0.5.0 to make sure that they
have that a "major ident change" milestone.

In case anybody wonders the obvious question:

I did a sweep through posixlib looking for posix constants as functions, i.e.
zero argument methods. dirent.scala was the only other file that needed
fixing. So, I think this PR completes the task.