Use guards instead of pragma once

[weshinsley]

With apologies to those who really prefer #pragma once, for consistency and to resolve a fairly uninteresting discussion, we're employing "code-owner's perogative" and saying we'll stay with header guards rather than #pragma once. While the "risks" of #pragma once are miniscule, there are no risks at all with the guards, other than preference, so for the sake of consistency, that's the way we'll go.

I appreciate that's not everyone's favourite thing, but it's a couple of lines per header - not important or prominent things, and there are many other more worthy things to focus in...