export and document sb-thread:join-thread-problem

[Benjamin]

The manual mentions that this function can be used to distinguish between different errors signaled by sb-thread:join-thread, but it isn't currently exported.

I discussed this a bit with stassats on IRC, and they pointed out that it is possible to distinguish between abort and timeout by setting the ':default' argument on 'join-thread' and then checking the second return value. I think it's still worth exporting 'join-thread-problem', because it was already referenced in the public docs, and the ':default' mechanism does not allow user code to distinguish the ':self-join' and ':foreign' cases.