I would like to change bio to do all I/O through a user replaceable 
function. The attached patch does just that, adding a new function Biofn() 
to register such a function, which replaces read/write.

Main uses I have in mind:

1. Allow programs to flush output buffers before stalling for more input, 
which would most likely be activated with a command line flag.

2. Use bio with things that are not file descriptors.

3. Apply filters to input/outputs (arguably could also be done with 
pipes).

4. Use bio with ioprocs.