client: Fix spelling errors found by codespell

[cjwatson]

My plan is to fix all the reported errors and then add codespell to make static-analysis; like any such tool it's imperfect, but I've generally found its maintainers to have pretty good judgement. This is just a small initial PR to check that you're OK with using it in principle; if so then I'll send a larger PR (~2500 lines) to fix all the remaining errors and enable it as part of make static-analysis. (If you'd rather not have it as an explicit lint step, then I guess most of the typos will be worth fixing anyway, so I can still send those.)

[stgraber]

Thanks! I believe we have golangci-lint running misspell already but that's definitely missed a whole bunch of typoes over the years, so likely not as comprehensive as codespell :)