Fix bug 424: doing full_read breaks things like cat which should return a

Fix bug 424: doing full_read breaks things like cat which should return a
chunk of data when they get it and not block until they've buffered 4k.

The use case was cat /proc/psaux, but you can also reproduce this by
running non-busybox cat by itself and typing things at the command line.
Then run busybox cat.  Notice how cat is _supposed_ to echo each line back
to us as we hit enter?