Warn about host possibility of being rooted by a DNS spoofer

When the installer runs the given command blindly, without looking at the payload being `curl`ed, an attacker could run any command as root by spoofing the host's DNS.

SSL may not prove save at this point, as the local cert chain is not always kept under a keen eye and may contain certs from questionable or outdated CAs.

Saving it temporarily to a file, looking at it and running it manually may be an option for the healthy paranoid admin.

Better warn the careless and let curl blerch when SSL smells.