dhcp-server: simplify dhcp server unref call

No need to use HASHMAP_ITERATE when we destruct all entries anyway.
coredump: optionally store coredumps on disk, not in the journal

Introduce a new configuration file /etc/systemd/coredump.conf to
configure when to place coredumps in the journal and when on disk.

Since the coredumps are quite large, default to storing them only on
disk.
tmpfiles: automatically clean up /var/lib/systemd/coredump after 3d
coredump: make sure coredumpctl can handle externally stored coredumps
coredump: add new "info" verb to coredumpctl showing detailed information about a coredump
coredump: add 3 more metadata fields to coredump entries