libsel4utils: Remove bitrotted client_server_vspace

This vspace implementation has not been maintained as the vspace interface has changed and
is now functionally incorrect. Due to it not being in use it is being deleted.

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

libsel4utils: Formatting fixes to README.

When Bitbucket/Github sees Markdown indented by four spaces, it treats it as
code. This was causing these bullet lists to look a little odd.