Make mssql dbapi module selectable

[Former user]

Not a bug report, but a feature addition.

mssql chooses adodbapi as its dbapi module if it's installed, and falls back to pymssql only if adodbapi is not importable. The two behave differently in significant ways. The attached patch adds module-level functions "use_adodbapi" and "use_pymssql" to allow the user to choose one of the two at runtime.

In addition, I've added "use_pyodbc" to use pyodbc as an (experimental) backend - so far, it seems to work pretty well out of the box (as far as I can tell, given that most of the test suite fails on mssql anyway).

[Former user]

(original author: ram) Committed in rev 2317, thanks

[paj]

This has introduced a bug in that the adodbapi-specific code in MSNVarchar and MSUnicode is no longer reached. Here is a patch to fix that issue.

[paj]

Fix for adodbapi problem

[Former user]

(original author: ram) The additional fix was committed in rev 2322, go, Paul go!