Use stat() instead of opendir() for checking existence of a dir in sftpHelpers

[harriha]

Original problem: opendir() fails if given path doesn't have read permissions for
the current user (permissions 0xx).

I'm working with a hosted environment and can't naturally access the root folder. Since the helper function checks existence of directories starting from the root ('/') via trying to open each dir, using sftp with createDirectories=true failed for me. Using stat() instead of opendir() seems to solve the problem, to me it feels like a better way for checking whether a dir exists or not. However, please note that I'm not 100% sure if there were something else behind the way how this helper function is implemented, thus review & testing welcome.

To clarify, the failing scenario with current version can be created as follows:

• Create a sample dir:

$ pwd
/path/to/home
$ mkdir private
$ chmod ugo-rxw private/

• Try to sftp nested file structure to /path/to/home/private with createDirectories=true
• -> Operation will fail

[andrewrjones]

Thanks for the patch! It looks good to me, and seems to work fine in my tests.