Make "setOpened" private, let gixsql assume full ownership of the cursor state

[mridoni]

It basically depends on who (the driver or the "main" library) has ownership of the cursor's state. I guess the main library should have and set it appropriately depending on the return code from cursor_open/close_cursor. At that point setOpened can be made private in Cursor and removed from the "interface" in ICursor, since the drivers will no longer be using it.

Originally posted by @mridoni in #110 (comment)

[mridoni]

046dfa9 should fix this