Retrieve Internet explorer proxy settings from the registry

I just re-wrote a library to easily retrieve the proxy settings of Internet Explorer (self answered SO question: https://stackoverflow.com/q/41764614/3207406).

Git repository

GoDoc (with an example)

```
package ieproxy

import (
"os"
"strings"
"sync"

"golang.org/x/sys/windows/registry"
)

// StaticProxyConf containes the Windows configuration for static proxy
type StaticProxyConf struct {
// Is the proxy active?
Active bool
// Proxy address for each scheme (http, https)
// "" (empty string) is the fallback proxy
Protocols map[string]string
// Addresses not to be browsed via the proxy (comma-separated, like linux)
NoProxy string
}

// AutomaticProxyConf contains the Windows configuration for automatic proxy
type AutomaticProxyConf struct {
URL string // url of the .pac file
}

// WindowsProxyConf gathers the Windows configuration for proxy
type WindowsProxyConf struct {
Static StaticProxyConf // static configuration
Automatic AutomaticProxyConf // automatic configuration
}

type regeditValues struct {
ProxyServer string
ProxyOverride string
ProxyEnable uint64
AutoConfigURL string
}

var once sync.Once
var windowsProxyConf WindowsProxyConf

// GetConf retrieves the proxy configuration from the Windows Regedit
func getConf() WindowsProxyConf {
once.Do(parseRegedit)
return windowsProxyConf
}

// OverrideEnvWithStaticProxy writes new values to the
// http_proxy, https_proxy and no_proxy environment variables.
// The values are taken from the Windows Regedit (should be called in init() function)
func overrideEnvWithStaticProxy() {
conf := getConf()
if conf.Static.Active {
for _, scheme := range []string{"http", "https"} {
url, ok := conf.Static.Protocols[scheme]
if !ok {
url, ok = conf.Static.Protocols[""] // fallback conf
}
if ok {
os.Setenv(scheme+"_proxy", url)
}

As is, the (my) code is really hard to test (needs actual registry modifications or env variable edition).

To ease the testing, one could change some functions:

func overrideEnvWithStaticProxy(conf ProxyConf, setenv envSetter) {
    ...
}
type envSetter func(string, string) error

func parseRegedit(regedit regeditValues) ProxyConf {
   ...
}

With this change overrideEnvWithStaticProxy and parseRegedit become self contained and can be easily tested!

This incurs a minor rewrite of getConf (which does only some plumbing):

func getConf() ProxyConf {
    once.Do(writeConf)
    return windowsProxyConf
}

func writeConf() {
    regedit, _ := readRegedit()
    windowsProxyConf = parseRegedit(regedit)
}

StackExchange Code Review Q#153143, answer score: 2