Add a test to check that read/write return an error

(per http://byterunner.com/16550.html and manually tested in qemu bare metal echo)

and fix a few exec() bugs

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

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

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

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

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

This patch does the following:

- Add .text.* to the .text section in the output
- Add an assertion that the trampoline does not overflow a page
- Add the .rodata section
- Make .sdata and .sdata.* (which is for small data) be absorbed into
  the .data section, because we don't need to distinguish between them;
  this prevents .sdata from appearing in the output
- Make the analogous change for .srodata and .sbss
- Make all the data sections 16-byte aligned

This patch also updates the .editorconfig for *.ld files.