lei_input: drop "From " line on single "eml" (message/rfc822)

lei_input: drop "From " line on single "eml" (message/rfc822)

This matches the long-standing behavior of public-inbox-mda,
public-inbox-learn and our other tools.  It is useful because
mutt, "git format-patch", and likely other tools will
pipe a single message with a "From " header line, but with
no further "From " escaping or Content-Length: header.