Fix MSSQL index and foreign key names.

First, the index names in MS SQL, as in MySQL, are only unique per
table, whereas they need to be globally unique (per database) in
PostgreSQL. So reuse the infrastructure we had for MySQL here.

Second, the way we trick table names in index and fkey structures means
that we already did quote the names and we don't want to quote them
again, so add a new possible *identifier-case* value to handle the case
where nothing is to be done, pretty please.