coredump: Add --all option

coredump: Add --all option

This option has coredumpctl look at all journals instead of only the
local ones. This allows coredumpctl to show information about remote
coredumps if the coredumps are made available in /var/lib/systemd/coredump
and the corresponding journals are made available in /var/log/journal.

This is already possible using the --directory option but --all makes it
more user friendly since users don't have to enter the journal directory
anymore as long as it's available under /var/log/journal.