all: use HTTPS for links that support it

Many websites now support HTTPS that may not at the time the code was
committed; let's use the HTTPS links where we can.

Change-Id: I7099dfa0dbb213294e65b4387f343d6e8f955b97
Reviewed-on: https://go-review.googlesource.com/47131
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
22 files changed