Non-unique thread spawns

[sim642]

In some special cases it's desirable to spawn threads as non-unique right off the bat:

1. Unknown functions which spawn create unique threads: Clean up some special functions #837 (comment). This is unsound and we should instead create the new thread immediately as non-unique.
2. Klever stubs have a non-unique thread spawning function: Klever concurrency safety support #688 (comment).

An extra uniqueness argument needs to be added and passed from ctx.spawn to the threadid analysis through the numerous transfer functions involved.

[michael-schwarz]

Unassigning myself as I currently do not plan on working on this the next few months.