One goal is now achieved: Different types of ISDN net devices now have
a struct ops which describes them, so we don't have a mess of
if (lp->p_encap == <whatever>) everywhere, but things even nicely
split into

isdn_net.c: Common stuff and ethernet, raw-ip, and similar
isdn_ciscohdlck.c: Cisco HDLC + keepalive
isdn_ppp.c: Sync PPP

where common code to be used library-like is provided by isdn_net.c