all: fix typos

Use the following (suboptimal) script to obtain a list of possible
typos:

  #!/usr/bin/env sh

  set -x

  git ls-files |\
    grep -e '\.\(c\|cc\|go\)$' |\
    xargs -n 1\
    awk\
    '/\/\// { gsub(/.*\/\//, ""); print; } /\/\*/, /\*\// { gsub(/.*\/\*/, ""); gsub(/\*\/.*/, ""); }' |\
    hunspell -d en_US -l |\
    grep '^[[:upper:]]\{0,1\}[[:lower:]]\{1,\}$' |\
    grep -v -e '^.\{1,4\}$' -e '^.\{16,\}$' |\
    sort -f |\
    uniq -c |\
    awk '$1 == 1 { print $2; }'

Then, go through the results manually and fix the most obvious typos in
the non-vendored code.

Change-Id: I3cb5830a176850e1a0584b8a40b47bde7b260eae
Reviewed-on: https://go-review.googlesource.com/c/go/+/193848


Reviewed-by: Robert Griesemer <gri@golang.org>