Drop "Copyright abandonded" header

Add CC0 as the license. SPDX does not have a "public domain" tag, but CC0 is
more or less equivalent. We should have *some* header to avoid doubts in the
future.

tools/check-includes.pl

@@ -1,7 +1,7 @@
+# SPDX-License-Identifier: CC0-1.0
 #!/usr/bin/env perl
 #
 # checkincludes: Find files included more than once in (other) files.
-# Copyright abandoned, 2000, Niels Kristian Bech Jensen <nkbj@image.dk>.
 
 foreach $file (@ARGV) {
 	open(FILE, $file) or die "Cannot open $file: $!.\n";