Sigh. Apparently "Internetwork analyzer" covers a multitude of sins;
both LAPB and PPP captures get written out with that network type.
Flag it as WTAP_ENCAP_UNKNOWN when the file is opened, and, when we see
the first packet, check whether the address field is 0xFF, in which case
we flag it as PPP, or anything else, in which case we flag it as LAPB.