Correctly support dot-stuffed header field names

[Julien-Elie]

Several bugs need fixing:

• innd does not correctly compute the :bytes metadata item (initial dots in header fields are counted whereas they should not);
• nnrpd does not correctly parse header field names with a leading dot in HDR, XHDR and XPAT commands;
• nnrpd does not return a valid dot-stuffed response in LIST OVERVIEW.FMT if it contains header field names with a leading dot;
• nnrpd accepts articles with bad dot-stuffing whereas it should reject them at injection time;
• nnrpd strips the first dot of header field names at injection time (instead of preserving it).