Various spelling/grammar tweaks and typo fixes.

Usually just a word or punctuation; occasionally I rewrote a sentence
if I had a hard time determining what was meant by the original. (N.B.
if I misunderstood something, then my re-wording will be wrong!)

Also added eol to many files that were missing it, which throws off
'cat somefile' at a shell prompt (but did not yet touch anything under
src/, which also has many files without eol).

I did not re-line-wrap the text I changed, so that the diffs are
minimized and meaningful changes are easier to spot.