Spelling fixes

Note that you may want to drop individual patches (especially things to third party libraries). You may also want to fold all commits of a given class into a single commit.

Commits are organized so that a single spelling fix is generally covered by one commit, this makes it easy to discard any one commit if it's objectionable (e.g. if it changes en-GB to en-US). Please don't feel any obligation to maintain the commit sequence, messages, contents, etc.