WinHTTP: Try to detect and use system configured proxy

Web Proxy Auto-Discovery (WPAD) protocol not supported so far.

Signed-off-by: Sven Strickroth <email@cs-ware.de>
Add WPAD and PAC support

Signed-off-by: Sven Strickroth <email@cs-ware.de>
Disable by default, but how to allow users of the library to enable this?!

Signed-off-by: Sven Strickroth <email@cs-ware.de>