Support for HTTP-Proxy

My Pull request adds functionality to use FreeRDP behind an HTTP proxy.
Enable the CMake option `WITH_HTTP_PROXY` to build.
There is a command line option `/http-proxy:<host>:<port>`.
For Connections using a gateway, also the `$https_proxy` / `%HTTPS_PROXY%` environment variables are respected.
Tested to work with squid and with TSG.