Diagnose programs where main exits before subthread

[margnus1]

In Pthreads, when the main thread exits, all threads in the process are terminated. This could lead to unexpected behaviour if some side-effect expected from the program is cut off. We should issue a warning if the main thread exits while not all subtreads have been joined.

We will probably need an opt-out switch for this diagnostic, as there could be legitimate uses of this behaviour, as well as benchmarks that expect the old nidhugg behaviour.