Fix all the whitespace

- remove trailing whitespace
- remove duplicate blank lines
- remove blank lines at end of files

Fix licenses

Move sel4_debug.h back into libsel4utils

This patch also renames a header {sel4debug/sel4_debug.h=>sel4utils/strerror.h}.

Additionally, convert some header guards to #pragma once and update
README.md to describe what things should go into libsel4utils and
libsel4debug.

SELFOUR-513