add a few safety checks in specfs because
now it's possible to get entries in [cd]devsw[] which are ALL NULL
so it's better to discover this BEFORE jumping into the d_open() entry..
more check to come later.. this getsthe code to the stage where I
can start testing it, even if I haven't caught every little error case...
I guess I'll find them quick enough..
This is a place for all things to do with conf.c and conf.h
that are not machine specific.
Other things that are at present in i386/isa/conf.c might
migrate into here..
It's the first small step in cleaning up the device interface
to make it more dynamic and to assist in more modular drivers
(i.e. both loadable via LKMs and linked in..
e.g able to add a device without having to edit conf.c)
this code is not yet used and the whole thing will be conditionally
compiled in for a while till proven useful :)
