and fix a few exec() bugs

might as well use 0.

since other calls (e.g. exec()) panic on out of memory.

fix a bug in fork() recovering from out of memory

The qemu syntax for a ram disk was documented incorrectly. The documented syntax is here:
https://qemu.weilnetz.de/doc/qemu-doc.html

fix bugs in read/write return values when there's an error

get rid of static for walk() and freewalk()