Use the https:// URLs to get syscalls.master for freebsd, openbsd and
dragonfly. For netbsd, keep using the http:// URL since HTTPS is not
supported.

Change-Id: Ibcbd19d5e3b4585552ea65dcdc742c2e7bfce944
Reviewed-on: https://go-review.googlesource.com/c/157057
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>