hdlc_Fcs
fcs = hdlc_Fcs(MBUF_CTOP(bp), bp->m_len);
extern u_short hdlc_Fcs(u_char *, size_t);
fcs = hdlc_Fcs(bufp, 2 + orglen);
fcs = hdlc_Fcs(bufp, wp->m_len = pp - bufp);