systemd: enable capturing coredumps for inspection via coredumpctl

Gentoo disabled this functionality because coredumpctl didn't exist yet.
It does now and is pretty slick so lets enable this. Dumps are stored in
/var/lib/systemd/coredump by default. Optionally they can be stored in
the journal by modifying /etc/systemd/coredump.conf
Merge pull request #748 from marineam/core

systemd: enable capturing coredumps for inspection via coredumpctl