REUSE: move copyright headers to .checksrc

To make it simpler to move them around, create and delete them without
syncing with `REUSE.toml`.

Also:
- checksrc: allow empty lines in `.checksrc`.
- comment on why curl printfs are disallowed in examples.

Closes #19024

scripts/.checksrc

@@ -1 +1,5 @@
+# Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
+#
+# SPDX-License-Identifier: curl
+
 allowfunc printf

scripts/checksrc.pl

@@ -202,6 +202,10 @@ sub readlocalfile {
         if(/^\s*(#.*)/) {
             next;
         }
+        # Skip empty lines
+        elsif($_ eq '') {
+            next;
+        }
         elsif(/^enable ([A-Z]+)$/) {
             if(!defined($warnings_extended{$1})) {
                 print STDERR "invalid warning specified in .checksrc: \"$1\"\n";