Sqitch MS SQL Server to ODBC.

Because otherwise one would need to have Sybase libraries to build
DBD::Sybase. Also, make URI::mssql the base class and have URI::sqlserver
inherit from it, rather than the other way around.

Since ODBC stuff is nearly always the same for the DBI interface, add
URI::_odbc as an abstract subclass for ODBC-connected database URIs to use --
at this point URI::mssql and URI::vertica.