SqlServer: support single sign on / Windows credential authentication
This is done through a new db-extra-arguments section in instance_dir/sources
which is passed as a string to the extra_args named argument of
lgc.db.get_connection. If this argument is Trusted_Connection, windows
authentication is used instead of login/password.
This change requires at least revision 5475ec3f6412 of logilab.common which
will be included in logilab-common 0.46. debian/control updated to mention this
new dependency.
SQL Server port: convert Password to Binary
This conversion is required because pyodbc uses buffers for Binary fields
and won't be able to cast a string/unicode value to the db VARBINARY column
type otherwise.