proxy auth

Simplifies proxy code to use the existing libwebsocket_set_proxy.

Enables libwebsocket_set_proxy() to parse username:password@ at front of
servername in both http_proxy and info->http_proxy_address.

If given the base64 version of the credentials are sent in the CONNECT
header to the proxy.

Port is now taken from info->http_proxy_address server:port syntax, but if
a port is given in the now deprecated info->http_proxy_port (ie, is nonzero)
then it is allowed to be missed out and the info port used instead for
backwards compatibility.

Signed-off-by: Andy Green <andy.green@linaro.org>