mk-ca-bundle.pl: first, try downloading HTTPS with curl

As a sort of step forward, this script will now first try to get the
data from the HTTPS URL using curl, and only if that fails it will
switch back to the HTTP transfer using perl's native LWP functionality.
To reduce the risk of this script being tricked.

Using HTTPS to get a cert bundle introduces a chicken-and-egg problem so
we can't really ever completely disable HTTP, but chances are that most
users already have a ca cert bundle that trusts the mozilla.org site
that this script downloads from.

A future version of this script will probably switch to require a
dedicated "insecure" command line option to allow downloading over HTTP
(or unverified HTTPS).