Release notes: use SPONSORS (no file type) as the published file name

SPONSORS.txt is the website published filename, but it seems some distros
were using the source code file name.

Use SPONSORS.ist for the coded listings instead.

Makefile.am

@@ -43,7 +43,7 @@ EXTRA_DIST = \
 	INSTALL \
 	QUICKSTART \
 	README \
-	SPONSORS.txt \
+	SPONSORS \
 	bootstrap.sh \
 	po4a.conf
 

bootstrap.sh

@@ -149,7 +149,7 @@ do
 done
 
 # Make a copy of SPONSORS we can package
-sed -e 's/@Squid-[0-9\.]*://' <SPONSORS > SPONSORS.txt || (rm -f SPONSORS.txt && exit 1)
+sed -e 's/@Squid-[0-9\.]*://' <SPONSORS.list > SPONSORS || (rm -f SPONSORS && exit 1)
 
 # Fixup autoconf recursion using --silent/--quiet option
 # autoconf should inherit this option whe recursing into subdirectories

mkrelease.sh

@@ -81,6 +81,7 @@ inst $tmpdir/CONTRIBUTORS	$dst/CONTRIBUTORS.txt
 inst $tmpdir/COPYING		$dst/COPYING.txt
 inst $tmpdir/COPYRIGHT		$dst/COPYRIGHT.txt
 inst $tmpdir/CREDITS		$dst/CREDITS.txt
+inst $tmpdir/SPONSORS		$dst/SPONSORS.txt
 inst $tmpdir/ChangeLog		$dst/ChangeLog.txt
 if [ -f $tmpdir/doc/release-notes/release-$RELEASE.html ]; then
     cat $tmpdir/doc/release-notes/release-$RELEASE.html | sed -e '