[libpng16] Add PD license to new "reindent" tool; remove unused environment var

I didn't intend to distribute this but it leaked into the libpng16 source,
so here it is.

contrib/tools/reindent

 #!/bin/sh
 
-# reindent inputabsize outputabsize inputcontinuestring outputcontinuestring
-# eg, to change libpng coding style from 3-space indentation to 4-space
-#
-#  reindent 3 4 " " "  " < example.c.orig > example.c
+# reindent a libpng C source
+
+# COPYRIGHT: Written by Glenn Randers-Pehrson, 2016.
+# To the extent possible under law, the author has waived all copyright and
+# related or neighboring rights to this work.  This work is published from:
+# United States.
+
+# Usage:
+# reindent inputtabsize outputtabsize inputcontinuestring outputcontinuestring
 #
 # Assumes that continued lines begin with indentation plus one space, and
 # that continued comments begin with indentation plus " *".
+#
+# eg, to change libpng coding style from 3-space indentation to 4-space
+#
+#  reindent 3 4 " " "  " < example.c > example.c_reindented
 
-unexpand --first-only --t $inputtabsize $1 | \
+unexpand --first-only --t $1 | \
    sed -e "/^	$3[^\*]/{s/	$3/	$4/}" | \
    expand -t $2