coredumpctl: report corefile presence properly

coredumpctl: report corefile presence properly

In 'list', show present also for coredumps stored in the journal.

In 'status', replace "File" with "Storage" line that is always present.
Possible values:
   Storage: none
   Storage: journal
   Storage: /path/to/file (inacessible)
   Storage: /path/to/file

Previously the File field be only present if the file was accessible, so users
had to manually extract the file name precisely in the cases where it was
needed, i.e. when coredumpctl couldn't access the file. It's much more friendly
to always show something. This output is designed for human consumption, so
it's better to be a bit verbose.

The call to sd_j_set_data_threshold is moved, so that status is always printed
with the default of 64k, list uses 4k, and coredump retrieval is done with the
limit unset. This should make checking for the presence of the COREDUMP field
not too costly.