Improve the parsing of folded Xref header fields

[Julien-Elie]

I once began a patch (for 2.5.x) to handle folded Xref header fields after a bug report, but somehow not finished it.
It could be useful in theory as RFC 5536 allows folding in Xref header fields. Though this is not a common practice, and INN does not generate internally such folded headers.

Also, it enhanced skip_cfws() and skip_fws() to check for a continuation line. It may be the reason why I did not finally commit it (a thorough review of these calls should be done).

I put here what I came up with (for innd, nnrpd, expire, tradspool):

• folded-Xref.patch.txt