Change default mode for new directories created by the Filesystem class.

We could go back and forth about security practices and bring Github down. However, would it be more sane to change the default mode for for new directories to 755?

Yes, this may be one of those "let's help the newcomers" of a PR, but I think it would save most of us a couple keystrokes. Hopefully I'm not alone in that I rarely ever need to create new directories writable by users other than the one executing the PHP.

Sent to master like asked in #3894.