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

djgpp/README.in

@@ -179,7 +179,7 @@ along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 	This will install the products into your DJGPP installation tree given
 	by the default prefix "/dev/env/DJDIR".  If you prefer to install them
-	into some other directory you will have to set prefix to the appropiate
+	into some other directory you will have to set prefix to the appropriate
 	value:
 	  make install prefix=z:/some/other/place
 

djgpp/config.bat

@@ -471,7 +471,7 @@ redir -e /dev/null rm %XSRC%/po/stamp-cat-id
 
 Rem Update the arguments file for the configure script.
 Rem We prefer without-included-gettext because libintl.a from gettext package
-Rem is the only one that is garanteed to have been ported to DJGPP.
+Rem is the only one that is guaranteed to have been ported to DJGPP.
 echo --enable-nls --without-included-gettext >> args
 goto ConfigurePackage