Spell fixes by `codespell`

[inkdot7]

For someone wanting some fun with git diff --word-diff and other such games, the nice tool codespell can be used to clean up some misspellings in the sources.

git diff --word-diff is needed to check the results, since not all changes are good. But experience from other places is that it is quite rewarding. The options -L and -w, as well as -i 2 of codespell are useful to look up to.

One does not need to deal with the entire repository at once.