coredump: include stacktrace of coredumps in the log message

elfutils' libdw is maintained, can read DWARF debug data and appears to
be the library of choice for generating backtraces today.
coredump: simplify how we apply extended attributes to coredumps
coredumpctl: introduce new -1 switch for showing a single, most recent entry only

"coredumpctl info -1" is now incredibly useful for showing the most recent
stacktrace.
update TODO
journald: make SplitMode=uid the default

Now that we actually can distuingish system and normal users there's no
point in taking session information into account anymore when splitting
up logs.

This has the beenfit with that coredump information will actually end up
in each user's own journal.
update TODO