Add support for S390 dump files

Support single-volume basic features of an S390 dump.

Signed-off-by: Petr Tesarik <ptesarik@suse.cz>
S390 dump: Check end marker

Each complete dump ends with a dump marker. Check that it is present
and looks sane.

Signed-off-by: Petr Tesarik <ptesarik@suse.cz>