replacing all ifndef with pargma once

All the kernel header files now use pargma once rather than the ifndef,
as the pre-processed C files do not change while header files
are protected with pargma once. This will also solve any naming issues
caused by ifndef.

Convert license tags to SPDX identifiers

This commit also converts our own copyright headers to directly use
SPDX, but leaves all other copyright header intact, only adding the
SPDX ident. As far as possible this commit also merges multiple
Data61 copyright statements/headers into one for consistency.

style: consistently attach return type

Add attach-return-type to astyle

x86: Have single makeUserPDEInvalid function

makeUserPDEPageTableInvalid and makeUserPDELargePageInvalid were both just creating the same
kind of thing, an invalid mapping, which really should be thought of as the third type of
PDE entry type.

Fix licenses (the rest)

s/D61/DATA61/ in license headers for consistency

x86: Refactor generic x86 pte/pde manipulation

Previously generic functions for manipulating page table and page
directory entries were placed in mode/structures.h. These are moved
to a mode/vspace.h instead so that they can use existing functions
defined in arch/vspace.h.

The x86_make_pde_mapping function is dropped in this move as it is
not used. Instead it is replaced with a function for creating an
empty mapping for whatever is the vspace root.