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

example/ssh2_agent_forwarding.c

@@ -1,4 +1,5 @@
-/*
+/* Copyright (C) The libssh2 project and its contributors.
+ *
  * Sample showing how to use libssh2 to request agent forwarding
  * on the remote host. The command executed will run with agent forwarded
  * so you should be able to do things like clone out protected git