Reconsider setting termination handlers

[msimberg]

Setting the termination handlers does have value because we can print e.g. stacktraces which are not printed by default on segfaults, pika thread-specific information etc.

On the other hand, when pika is embedded in an application behind multiple other libraries, combined with other runtimes etc. who is to say that pika's termination handler is the termination handler to install?

Some in-between solutions would be to allow users to disable/enable pika's termination handler explicitly (with config options or explicit function calls).

[msimberg]

Possibly make this optional, off by default?