basic/journal-importer: "trusted" fields in binary format are not supported

basic/journal-importer: "trusted" fields in binary format are not supported

The parser never accepted "__"-prefixed fields in binary format, but there was
a comment questioning this decision. Let's make it official, and remove the
comment.

Also, for clarity, let's move the dunder field parsing after the field
verification check. This doesn't change much, because invalid fields cannot be
known special fields, but is seems cleaner to first verify the validity of the
name, and then check if it is one of the known ones.