Update documentation on core dumps

[julianbrost]

Our documentation on core dumps would benefit from some updates.

Current documentation: https://github.com/Icinga/icinga2/blob/master/doc/21-development.md#core-dump

Tasks:

• Verify if "The Icinga 2 daemon runs with the SUID bit set. Therefore you need to explicitly enable core dumps for SUID on Linux." is still up-to-date (I don't think so)
• Mention systemd coredumpctl
• Check sysctl kernel.core_pattern on all distributions we support and document the findings (i.e. which ones use coredumpctl, are there other similar tools, etc.)