sel4utils: Strfault: Preprocess out indices which are HYP specific

sel4utils: new new function: sel4_strfault()

* Returns a string for an seL4 fault type

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