I hacked radiusd to look at NAS-Port-Type. If (NAS-Port-Type == Async), then
(merit) radiusd checks against /etc/shells, otherwise it checks against
/etc/isdnshells (normal ppp users have /bin/nologin as their shell,
isdn users have /bin/isdn). I don't offer shell accounts so this works
well for me. If you do offer shell accounts, you could do something
similiar by checking GID instead of shell. 4 or 5 lines of code.