Misconfiguration no longer detected

With https://github.com/rust-lang-nursery/multirust-rs/commit/c3679a5922e68c5b08d7ff544f4d76031887523d the proxy test was removed. While I understand the rationale for it's removal, I do think it was useful:

1. On windows, the system-wide PATH has a higher precedence than the user-specific PATH, so by default, any existing rust install will silently override the rustup install, and the invocations of rustc will not be correctly proxied. Given how likely this is to occur, we should at least warn when this is happening.

2. Previously, multirust would intercept commands of the form <proxy> --multirust and print a message indicating that said proxy is working correctly. This was a useful feature in itself, and does not cause erroneous warnings.

There are a couple of ways this could be solved:

1. Only check the proxies on first install.
2. Provide a command to check that the proxies are correctly configured, and recommend using it in the post-install message.

Review commit c3679a5922e68c5b08d7ff544f4d76031887523d and the removed proxy test first. Reproduce the Windows PATH precedence case and compare it with the former <proxy> --multirust behavior. The issue is complete when the chosen approach reliably identifies or checks misconfigured proxies and communicates the result during installation or through a command.