Propagate signals from worker to its children

[Kobzol]

This is an alternative to #446.

Instead of using DEATHSIG, we create a background handler in the worker which propagates any received SIGTERM, SIGINT or SIGHUP signals to the worker children (=running tasks).

Advantages:

• Doesn't suffer from the performance regression caused by setting DEATHSIG.
• Propagates signals to the whole process group, which isn't done by DEATHSIG (that only handles the end of the parent process).

Disadvantages:

• A bit hacky (?).
• Doesn't handle SIGKILL.

Fixes: #416
Fixes: #431