lei import: ignore Status headers in "eml" messages
Those headers only have meaning with for mboxes. Don't surprise
users by trying to make sense of a header that is defined for mboxes.
It's possible to send email with (Status|X-Status) headers and
have those headers show up in a recipient's IMAP mailbox.
This was bad because an IMAP user may want to import a single
message through their MUA and pipe its contents to "lei import"
without noticing a mischievious sender stuck "X-Status: F"
(flagged/important) in there.