All: Check 'must', 'should', 'may' and replace with MUST etc., or other words

[Nat Sakimura]

The title says it all. We MUST NOT use 'must', 'should', 'may' etc.

[Michael Jones]

I actually just did this as part of my checkin for issue #838. See change set https://bitbucket.org/openid/connect/commits/36f209b01e94, which has over 200 changes. If anyone finds any specific RFC 2119 corrections that I've missed, please feel free to list them and reopen this issue.