Bring code into line with comment and ensure

Bring code into line with comment and ensure
that carr_freelist always has a carrier on
it. This should fix a problem where carr_freelist
has become NULL and then AdvISR() attempts to
put a carrier back on the list without checking
if carr_freelist is non-NULL.

This and the previous two changes to adwlib.c
and the change committed to adw.c at the same
time were derived in whole or part from work
done by/with dante@ from NetBSD.