

io operations are performed upon iodev_t's.
Some iodev_t's implement directories, so can respond
to the lookup operation, for example.

An iodev_t is created by calling a class-specific constructor;
for example "iodev_t *create_dir(char *name);" is onesuch.
All constructors return an iodev_t *, acquired from devsw.

namei performs the generic name lookup service; extracting
each path component, and calling the iodev_t method 'lookup'.

iodev_t's are placed into directories via the 'link' method.
