Add SQL_WCHART_CONVERT for UnixODBC. 

The simple test in testunicode.py fails otherwise, on Ubuntu 64 i686, with a 64 bit driver. (Virtuoso)
This is not yet final code, I will package it better once I hear confirmation that it does not break other configurations, or create tests otherwise.
I'm sending it as a pull request in this state to raise the issue.