coredump: get rid of _META_MANDATORY_MAX

coredump: get rid of _META_MANDATORY_MAX

No functional change. This change is done in preparation for future changes.
Currently, the list of fields which are received on the command line is a
strict subset of the fields which are always expected to be received on a
socket. But when we add new kernel args in the future, we'll have two
non-overlapping sets and this approach will not work. Get rid of the variable
and enumerate the required fields. This set will never change, so this is
actually more maintainable.

The message with the hint where to add new fields is switched with
_META_ARGV_MAX. The new order is more correct.