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
187d89bb07