Change Open/Close VIs for SFTP File and SFTP Directory classes to polymorphic

[volks73]

To be consistent with the Create/Destroy VIs of all other classes, the Open/Close VIs for the SFTP File and SFTP Directory classes should be changed to polymorphic, even if it is just one instance. This will provide a consistent look-n-feel for using the API and make usage more predictable.

There could be an argument made for renaming these to Create/Destroy as well. I believe I (internally) debated this for awhile and decided to keep the "Open/Close" nomenclature to avoid confusion with creating a file or directory on a remote SFTP server.

[volks73]

Closed as of 9ec48a9.