Hopefully should fix ghc ticket #4982.
The problem was permissions on directories: previously we used ordinary
createDirectory and on unix this creates dirs using the current user's
umask. If the root user has a silly umask then someone doing sudo
install will end up with dirs that are not readable by non-root users.

So the solution is to do the same as we do with files: override the
umask and explicitly set the file permissions based on the kind of
file: ordinary file, executable file and now also directory.

Sadly we also had to re-implement createDirectoryIfMissing to use our
new createDirectory wrapper function.