8032220: Files.createDirectories throws exception with confusing message for root directories that exist

[Alan Bateman]

This one came up on nio-discuss a few days ago.

The issue is that Files.createDirectories fails on some platforms when 
called with a path to a root directory that exists. The exception (which 
is misleading) happens on OS X and Windows (Linux and Solaris are okay).

On OS X then the issue is that mkdir fails with EISDIR when you attempt 
to use it to create /. On Windows it's because the win32 CreateDirectory 
fails with an ERROR_ACCESS_DENIED for this case. I've updated both 
implementations to better handle this case so that createDirectory 
throws the expected FileAlreadyExistsException. I've also tweaked the 
exception throw by createDirectories so it's less confusing for the case 
that the root directory is not accessible. The webrev with the changes 
is here:

http://cr.openjdk.java.net/~alanb/8032220/webrev/

-Alan.