mshtml: Use the last colon in proxy url as port separator

[Juan Lang]

> Is the ProxyServer specified as an URL? If yes then use the proper API
> to dissect the URL instead of cobbling something together.

Just to follow up in this idea, you could use InternetCrackUrl.
mshtml delay loads wininet, but it also loads urlmon, which also loads
wininet, so in effect wininet is always loaded and available.

But a larger question is, why load the proxy settings from the
registry only?  If you query wininet for them instead, then the
environment variable http_proxy is also checked.  There was bug 5625
about this, which I set to fixed based on commit
80f02b82d68902f32578a7bcf6cfbaa715b724ce.  I may have been mistaken.
Under what circumstances is the network.proxy.http gecko setting still
being used?  Maybe for embedded content, e.g. IMG tags?  If these were
set by querying wininet instead, then at least we wouldn't have
potentially different proxy settings for some HTTP requests than for
others.
--Juan