Post-Quantum Guidance for IETF protocols.

[[This is not an "official" work item anywhere, the -00 was proposed to TLS as such, but this version generalises to more than just TLS so is being proposed to the secdispatch list. The author does not expect that the current text will garner rough consensus, but wonders if we could get something useful that would without adding many words. The source for this is in https://github.com/sftcd/pqg/ PRs are welcome there too.]] Due to concerns about the possible future existence of a cryptographically relevant quantum computer (CRQC), additional IANA codepoints have been defined for algorithms that are hoped to remain secure even in the face of a CRQC. Adding code-points for to the relevant IANA registries often doesn't require IETF consensus. This means that anyone can register code-points for their favoured approach. In particular various government entities in various countries have made contradictory recommendations in this space, leading to potential confusion for those deploying applilcations using TLS. For example, anyone can register a PQ algorithm in the TLS registries with the RECOMMENDED column set to 'n' This document sets out a deliberately concise set of recommendations for typical uses of post-quantum algorithms. This assumes the reader is familiar with the topic. Some implementations and environments may have to meet other requirements that conflict with this guidance. Note that the audience for this document are those deploying systems now. This guidance is not aimed at those developing IETF protocols, nor implementations of those. Given that it seems that the latter groups (protocol developers and implementers) seem determined to define and implement almost every possible combination of PQ everything, those deploying systems now, that have such PQ all kinds of everything, can benefit from simple guidance that addresses the most important aspect of the PQ transition. It is quite likely this guidance will need to be updated with a short-ish period (perhaps about two years).