Add support for SQL Server Native driver

Since PHP 5.3 has been released, as per PHP documentation the mssql
driver is no longer available on Windows. Microsoft has published a
new SQL Server Native Driver.

This commits adds support for this new driver (sqlsrv) to MantisBT
as mssqlnative db type.

Thanks to genius_p and tomkraw1 for the original patch and testing.

Fixes 0011776