coredump: support tracing of secured cores via polkit

this touches a huge chunk of technology. the previous invocation
through `coredumpctl debug` has been removed entirely. instead we now
always prepare the debugger by extracting the core using `coredumpctl
dump`, depending on whether the actual core file is readable by drkonqi
directly that happens in-process, or via the polkit helper.

the polkit helper essentially does the same thing but in root scope.