For non-direct libstrongswan users, the deinitialization segfaults because
of the missing worker thread cancellation.

We want to avoid libvici users to get a cluttered stderr for no real error.

As we want to avoid the libstrongswan include dependencies for libvici, avoid
the use of the bool type. Unfortunately this change may break the ABI for
vici_dump(). As this function is mostly for debugging purposes, we do it
nonetheless; my apologies if somebody already relies on the ABI stability of
that function.

Even if the command actually can't fail, this looks more aligned to similar
commands.