Ensure *.core.in files are removed if user didn't intend to keep them

Unexpected abortion of rich-core-dumper can leave temporary *.core.in
files behind, which then take up device memory until manually removed.

In order to conserver space, *.core.in is named *.core.in.tmp during
the script execution and is eventually either deleted or, when
KEEP_UNSTRIPPED_CORE=true, renamed to *.core.in.

If the core dumper exits abnormally, the leftover *.tmp file is removed
after two days by already implemented cleanup mechanism.