sslstrip fix & don't restore iptables/ip_forward on exit when bettercap did not change them.

[SkyperTHC]

1. Do not restore ip_forward and iptables settings if they were not changed by bettercap (e.g. when set http.proxy.redirect false was used)
2. sslstrip was broken and did not handle these cases:

• Location redirects to https://foo.com:443 ended up as http://foo.com:443 and the victim's browser would try to open a cleartext HTTP connection to port 443. It should go to port 80 (the most likely HTTP port).
• The orig.Scheme get overwritten with https in http_proxy_base_sslstriper.go in the Request function. Thus the check for 'http' later will always fail (because it's not 'https').
• Response: always need to fix cookies (even during a 302/Location).
• Response: Must change all HTTPS to HTTP in cookies