Check if way-node index exists before enabling forward dependencies

[lonvia]

If a database is imported with --with-forward-dependencies=false and then updated without the option, updates will be slow to the point of being unusable because the way-node index is missing.

osm2pgsql should check that the index exists before allowing forward dependencies.

Related to #1311.

[joto]

We don't have the name of the index, but this should probably work:

SELECT count(*) FROM pg_indexes WHERE tablename='{prefix}ways' AND indexdef LIKE '%nodes%';

If the result is >0, the index is there.

[joto]

Do we need this check now that --with-forward-dependencies is going away anyway?

See #1866 .

[lonvia]

Nope.