all: Prefer https:// over http:// in source where possible.

This change cleans up as many internal web references as possible, to
make them use https. In some cases, the canonical references had
slightly different URLs in addition to the change in schema.

Sadly, git.savannah.gnu.org is still http-only.

• Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>