LCP: accept an auth nack to CHAP with length == 4
It seems at least some products send this:
{LCP (0xc021), length 12: LCP, Conf-Nack (0x03), id 9, length 10 encoded length 8 (=Option(s) length 4) 0x0000: c021 0309 0008 Auth-Prot Option (0x03), length 4: CHAP[|lcp]}
If we refuse to nack to CHAP, we'd stay stuck with PAP and eventually get a reject.