http*_proxy improvements

- HOST_MACHINE can be used in http*_proxy fields
  - expands to host machine's hostname
    - helps enable developers running local caches
    on their dev box using mdns or static ip/dns.
- proxy health checking: if the proxy isn't up,
  it won't be used. grumpy dev doesn't want to
  install a proxy… fine. not up? no problem.
- install.sh is sed'd to rewrite https link to http
  so we can cache the requests.
  - seems to get what's needed (despite only changing
    the first link that's redirected immediately).