Instead of indexing registered device contexts, use mandatory named port IDs

[vbmacher]

Devices are connected to each other using procedure:

1. context registration (device which allows connection)
2. context retrieval (device which wants to connect)

However connecting multiple devices brings challenges, because it is not clear which device actually requested connection. Thus, if e.g. both PTP and terminal devices are connected to 88-2SIO, the 88-2SIO doesnt know which is which. Also user in schema editor cannot choose this.

Thus, every device should register it's contexts by some ID and then connecting device should name that ID when retrieving context. Not-specifying ID should be allowed only if registering device has just one context.

As of multiplicity, it should be allowed to connect multiple devices to single context ID.