copyright: remove years from copyright headers

Also:
- uppercase `(C)`.
- add missing 'All rights reserved.' lines.
- drop duplicate 'Author' lines.
- add copyright headers where missing.
- enable copyright header check in checksrc.

Reasons for deleting years (copied as-is from curl):
- they are mostly pointless in all major jurisdictions
- many big corporations and projects already don't use them
- saves us from pointless churn
- git keeps history for us
- the year range is kept in COPYING

Closes #1082

tests/test_read_algos.test

@@ -1,6 +1,6 @@
 #!/usr/bin/env bash
 
-# Copyright (c) Viktor Szakats
+# Copyright (C) Viktor Szakats
 
 set -e
 set -u