Update the COPYING file (Only URLs have changed).

Update GPL headers in all files:
- Replace "http" with "https" in the GNU URL of all GPL headers.
- Use 80 character width limit.

Replace "http" with "https" in Ctags and Cscope URLs (in the README.md
file).

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