owut: add proxy support via env vars

[luizluca]

Fixes #54

[efahl]

From the conversation on the issue, seems like you've got the bases covered here, so let's clean it up a bit. So, grab that info about the proxy -> server variants and make a table in the commit message, something like (I want sufficient info so that I can write the documentation section in the wiki, without having to dig around).

The proxy and server URLs can use different protocols, here's
what's expected to work.

  http_proxy -> http server - should work with all proxy servers
  https_proxy -> https server ...

Also, if you have precedent references for this implementation, like links to how curl or wget handle this, include those in the commit message, too.

[luizluca]

From the conversation on the issue, seems like you've got the bases covered here, so let's clean it up a bit. So, grab that info about the proxy -> server variants and make a table in the commit message, something like (I want sufficient info so that I can write the documentation section in the wiki, without having to dig around).

There is no need for a table: all scenarios are fine. owut only uses https (but I forced it to test http as well) and both PROXY HTTP or HTTPS (I just tested it) should work, although PROXY HTTPS requires a valid certificate (or editing the code to not verify certs like I did in my tests). In the future, we could expose owut options to skip or set custom ca files, but it might not be forth it until we have support for basic authentication. I also added a note about it and I thing uclient ucode is also not ready for proxy auth yet.

Also, if you have precedent references for this implementation, like links to how curl or wget handle this, include those in the commit message, too.

We are limited by what uclient offers. I added a note that noproxy is not considered (just like uclient) but it is already over that uclient handles.

I dropped the use of HTTPS?_PROXY env vars as it is only used in legacy situations and we does not need to add it in something new.

I also renamed "proxy_url" to just "proxy" as it set_proxy_url might confuse someone reading the code. I'm not sure set_proxy_url() is very intuitive for what it does. "proxy_url" for uclient is "the URL I'll ask the proxy to access" and not "The URL to access the proxy".