Define appropriate behavior when attaching to a named connection

[SafirSDKImport]

It is possible to open connections with the same common part and instance part name in different contexts.

When attaching to a named connection there is no parameter for the context. In the current implementation the secondary connection will attach to the first created connection with a matching common part and instance part without considering the context. It must be decided if this is an acceptable behavior.

These are the alternatives:

1. The current behavior is good enough. In this case it should be documented in the code and User's Guide.
2. Don't allow connections with identical common part and instance part in different contexts.
3. Make it possible to give context when attaching to a named connection.

Reported by: anderswiden

Original SourceForge Ticket #224: safir/tickets/224