Since HTTPS is "the new normal", this update changes a lot of man page
examples to use https://example.com instead of the previous "http://..."
Closes#5969
Reported by the new script 'scripts/copyright.pl'. The script has a
regex whitelist for the files that don't need copyright headers.
Removed three (mostly usesless) README files from docs/
Closes#5141
Since the TCP keep-alive options were added in 705f0f7 the start and
interval default values have been 60, but that wasn't documented.
Bug: https://curl.haxx.se/mail/lib-2017-01/0000.html
Reported-by: Praveen Pvs
