Update the COPYING file (Only URLs have changed).

Update GPL headers in all files:
- Add "This file is part of Ondine."
- Replace "Ondine" with "This program" (to have similar headers among
  projects).
- Replace "http" with "https" in the GNU URL.
- Use 80 character width limit.

Signed-off-by: Cédric Marie <cedric@hjuvi.fr.eu.org>