cmd/go: for go get -insecure, skip TLS certificate checking

cmd/go: for go get -insecure, skip TLS certificate checking

The flag is already named -insecure. Make it more so.

If we're willing to accept HTTP, it's not much worse to accept
HTTPS man-in-the-middle attacks too. This allows servers
with self-signed certificates to work.

Fixes #13197.

Change-Id: Ia5491410bc886da0a26ef3bce4bf7d732f5e19e4
Reviewed-on: https://go-review.googlesource.com/18324
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>