maint: fix comment typos

Using http://github.com/lyda/misspell-check, massage its
output into sed commands to perform the suggested changes.
Initially, I filtered out the THRU->Through changes, because
that failed to retain capitalization in the grammar token.
Instead, do this manually, beforehand:

sed -i s/THRU/THROUGH/ tests/existing.at
git ls-files|misspellings -f -|perl -nl \
  -e '/^(.*?)\[(\d+)\]: (\w+) -> "(.*?)"$/ or next;' \
  -e '($file,$n,$l,$r)=($1,$2,$3,$4); $q="'\''"; $r=~s/$q/$q\\$q$q/g;'\
  -e 'print "sed -i $q${n}s!$l!$r!$q $file"'|bash

tests/local.at

@@ -619,7 +619,7 @@ main (void)
 # In some versions of Autoconf, AT_CHECK invokes AS_ESCAPE before
 # expanding macros, so it corrupts some special characters in the
 # macros.  To avoid this, expand now and pass it the result with proper
-# string quotation.  Assume args 7 thru 12 expand to properly quoted
+# string quotation.  Assume args 7 through 12 expand to properly quoted
 # strings.
 
 m4_if(m4_index(m4_quote($3), [no-xml]), -1,